Oriented around a north-east/south-west axis, and situated on a sometimes steep-sloping spur overlooking the River Kennet (bounding its south-east side), medieval Marlborough was advantaged by its road connections: an east-west route through it (High Street) led to Reading and London in one direction and Chippenham and Bath (and ultimately Bristol) in the other, while a north-south route connected to Salisbury, Winchester, and Swindon. It may be significant that it lay close to the site of the Roman fortified town of Cunetio – although continuity of outlying settlement in the vicinity of Marlborough during the Romano-British period appears minor and of rural character – and to Ramsbury, which in 909 became a centre of episcopal administration.

The earliest reference to Marlborough is in 1086, when the most common form of the name then (and later) recorded, Merleberge, has prompted various interpretations, such as that it refers to a 'marl bridge' – this perhaps being the chalk outcrop on which the High Street was carried across the site; but the one most widely accepted is that the suffix means not a town but a hill or barrow (see below), while the prefix may be either a personal name or a derivative of an Anglo-Saxon term meaning great. Having said that, there is a hint in Domesday of its urban status, in that the king had the third penny (one-third of the revenues from the place), something also due from Salisbury, Malmesbury, Cricklade, and Bath. Otherwise Domesday gives little detail that would help us characterize Marlborough, beyond reference to its church. However, its importance is further suggested by the fact a mint was transferred there from Great Bedwyn ca.1068 (the earliest Marlborough coins dating from the 1070s), and remained until the 1090s.

Jeremy Haslam [Anglo-Saxon Towns in Southern England, Chichester: Phillimore, 1984, p.98] theorizes that Marlborough's territory was carved out of the Saxon parish of Preshute, part of a royal hundred, and it may therefore have been a deliberate burh foundation by Edward the Elder, connected with the promotion of Ramsbury to an episcopal seat, and intended partly as a distribution and processing centre for the region's wool. Part of the common arable land later associated with the borough was known as Port Field and a mill on the Kennet as Port Mill (documented ca.1190) – names possibly echoing the commercial character of the settlement. Haslam also adduces other topographical evidence: as a potential Saxon marketplace, an open space (known by 1289 as The Green) adjacent to the church dedicated to St. Mary, crossed by a north-south street that could have been the original axis of a Saxon settlement, and from whose name (Herd Street) we might infer the movement of livestock from outside farms to market; and, as possible burh boundary, a second street named Kingsbury, running west of and parallel to the first and skirting the south-western side of the churchyard (a third parallel route, perhaps representing the eastern boundary, having the suggestive name Blowhorn Lane). A Newbury Street is also heard of, south of St. Mary's church, its position again suggesting a boundary of early settlement. The several north-south streets were connected by an east-west street that ran along the north side of The Green and the churchyard, but did not connect directly with the spinal street of the Norman suburb. Kingsbury and High Street were later treated as separate aldermannic wards of the borough.

However, Haslam allows, Marlborough's medieval layout was the outcome as much of expansion in the Norman period as of original burh planning. The Normans extended the existing settlement south-westwards, built a second church (dedicated to St. Peter) and a castle at the south-western end of this, and between St. Mary's and St. Peter's churches laid out a wide street, flanked by burgage plots, to serve as the new marketplace; market crosses were later installed near either end of this street, but commercial activity may have focused particularly at the east end, with concentrations of vendors of particular products developing in different parts. The burgage plots south of this High Street stretched down to the river, while those on the north side extended to a Back Lane. That the planted town did not extend into the area between St. Peter's and the castle was desirable for military and perhaps other reasons; however, there are indications of unplanned, and possibly short-lived, spread of settlement into part of this area, as the castle transitioned from military function to hospitality for royal visitors in thirteenth or early fourteenth centuries. The Norman 'colony' would have originated as a suburb if we accept the prior existence of a defended burh, but it has the characteristics of a new town foundation. Before mid-thirteenth century a further suburb known as the 'New land' had grown up, or perhaps been laid out, north-east of the Saxon core, its residents building a chapel for themselves in the 1250s; though part of an external tithing, for some administrative purposes it was treated from 1268 as a ward of the borough.

It is suspected a simple fortification was erected by William the Conqueror, either as he expanded his conquest into the west country in 1067/68 and then faced the west-coast threat from Harold's sons raiding from Ireland, or perhaps in conjunction with transfer of the mint, for we hear of the Bishop of the South Saxons held hostage at Marlborough in 1070. This tower, if such it was, was situated within a bend in the river, which offered additional protection (and later supplied water to the moat); it was positioned to control passage along the river and across its ford for the Bath-London road, while still having a view of the Saxon settlement around the crossroads and the other ford where the Salisbury road crossed the Kennet and entered the settlement. The notion that William used an existing prehistoric earthwork on which to raise the motte is dubious, though still has its proponents, inspired partly by the fanciful early interpretation of the town name as referring to a barrow where Merlin was buried [See D. Field et al. "The Marlborough Mount Revisited.", Wiltshire Archaeological and Natural History Magazine, v.94 (2001), 195-204].

It is thought that, at the beginning of the next century, Roger le Poer, newly-instituted Bishop of Salisbury, strengthened and expanded the castle, and it was perhaps at this time that Marlborough was extended with the borgo, as it would have been called on the continent, between Saxon Marlborough and the castle; this castle/colony/market combination was a typical Norman strategy for consolidating control of urban centres that were strategically located or considered past strongholds of Saxon power. The neighbouring forest was good hunting grounds and attracted monarchs to visit the area frequently. The castle was rebuilt in stone following the Anarchy, and in the early thirteenth century were added residential apartments, facilities for a provincial treasury, a hall large enough to host the parliament of 1296, and chapels. It was, in the later Middle Ages, neglected as a residence and allowed to fall into ruin.

Haslam's theory that Marlborough was established by Edward the Elder as a market town in conjunction with the elevation of Ramsbury to an episcopal seat, and prospered at the expense of Great Bedwyn, has appeal but, in contradistinction, a surprising lack of Saxon finds in Marlborough has archaeologists unable to demonstrate presence of a significant population in that period, while there is no documentary reference to Marlborough prior to 1086, even though the Domesday entry suggests a pre-Conquest settlement. This led the authors of the Extensive Urban Survey report to reject the theory and interpret Marlborough as a Norman foundation, with the Kingsbury neighbourhood a later extension of the Norman planted town near the castle – even though it is that area which incorporates crossroads, grid-pattern streets around an open area, and a candidate for the church mentioned in Domesday. This interpretation required that the Saxon origins of the town name be explained by the presence of a barrow, which itself is unsupported archaeologically. Furthermore, the name Kingsbury raises the possibility of a Saxon villa regalis there, as the centre of a larger royal estate. That 'Kingsbury' is also found at Wilton and Calne tends to favour the Late Saxon interpretation.

That the Normans chose to make Marlborough a base for their control of Wiltshire was probably not simply because of its strategic location within the communications system but also because they found an existing Saxon centre of population and commerce, with ties to the monarchy, that could usefully be dominated and developed – hence the introduction of castle and market-focused burghal component – while at the same time disempowering Bedwyn as a military/civil centre with even closer association to the Saxon regime. On those grounds, the origins date presented here aims for a reconciliation of the two interpretations; though hard to know at what point Marlborough could be considered truly urban – the original Saxon settlement perhaps having more of the character of a proto-urban port with a communications orientation towards Ramsbury – it seems likely that it was so prior to the transfer of the mint there (as much about weakening Bedwyn as strengthening Marlborough), and the subsequent foundation of a Norman castle-town which sought to reorient Marlbrough (with limited success) to the route connecting London and the west country.

The charter issued in 1204 by King John (who had held the castle when prince and wedded his first wife there) acknowledged that Marlborough was a royal borough and granted the burgesses an eight-day fair in August and markets on Wednesdays and Saturdays, with all pertinent customs already in effect at Winchester and Oxford, and various other privileges – such as exemption from various kinds of tolls and feudal impositions like scot-ales and brewingavel – that altogether amounted to grant of liber burgus status. The charter also confirmed the right to a merchant gild, for which the burgesses had paid £5 to the king in 1163; its hall is believed to have been situated at the east end of the High Street, not far from where a market house and a town hall stood in the post-medieval period. By 1239, when there was a dispute between Marlborough and Southampton over tolls being demanded at the latter from men of the former (on the grounds Southampton's borough charter pre-dated the Marlborough exemption), it appears that membership in the gild and burgess status were equivalent; Southampton agreed not to take toll from any Marlborough trader who took oath to being a gildsman. In 1229 the king granted a shorter November fair to be held in the Newland at Marlborough, although in practice this may have meant The Green, while a third fair (for June) was authorized in 1246 to be held in and around the churchyard of St. Peter's (which archaeology has shown to have been much larger than its present size) and said to be for the betterment of the town.

In March 1228 the burgesses took advantage of the one of the regular stays at Marlborough castle by Henry III, who had just emerged from his minority, to complain that the market at Ramsbury was proving damaging to theirs. This market had been licensed to the Bishop of Salisbury (Richard Poore) in 1219, to take place on Fridays. Under a general prohibition of markets and fairs raised during Henry's minority, sheriffs had been ordered to suppress any such, and this may have been carried out at Ramsbury. However, in March 1227 the Bishop (who was at this time one of the group working with Hubert de Burgh to govern the realm on Henry's behalf) obtained a letter to the sheriff of Wiltshire exempting his Ramsbury market and, two weeks later, received, in the context of a broader royal grant of privileges, the right to hold a market there on Tuesdays; later in the year he arranged to be excused the fee owing for the Friday market licence. It is perhaps surprising that, in view of Bishop Poore's position at the centre of government, the Marlborough complaint was successful in obtaining an order to the sheriff to suppress the Ramsbury market – presumably the Tuesday event was the target, even though the Friday occurrence must have been equally problematic for Marlborough's Saturday event. However, the grant of the Tuesday market had included the standard proviso of it not proving damaging to any other market, the young king was determined to assert his independence of action, and the energetic and powerful Bishop Poore was about to be sidelined, when in May he was translated to the see of Durham and left royal service. Under his less forceful successor, although not until 1240, a resolution was reached in the Marlborough/Ramsbury dispute through royal mediation: in exchange for a grant of several fairs at Ramsbury and another episcopal manor, Bishop Bingham renounced his market at Ramsbury, with the reservation that his tenants there continue to be allowed to sell bread, ale, fish, meat and perhaps other victuals at any time without challenge; the bishop was not permitted to call such activities a market, nor to impose tolls on these transactions, and they were not to focus around a particular day of the week. Whether this compromise concerned the Friday market, or both that and the Tuesday event (if its suppression had been long delayed by legal wrangling), we do not know. But a Tuesday market at Ramsbury was re-licensed in 1300, and this time without any evident opposition from Marlborough.

In 1275, in the context of the hundredal enquiries, Marlborough targeted Swindon with a complaint about damaging competition. William de Valence, it was alleged, had been operating a market there for fifteen years which was depriving Marlborough of some 40s. of profits per year (from tolls and court perquisites); Swindon was some distance away and the jurors would probably not have raised their concern had they not suspected its market was unlicensed. However, de Valence was able to produce a charter grant of Henry III and clear himself. Advantage of the hearings was taken for a further allegation that an unlicensed market was occurring at Ramsbury on Sundays, even though the king had prohibited it, also claimed to be costing Marlborough's market a similar (standard-looking) sum in earnings; this seems a dig at the victuals trade preserved by the 1240 agreement, which was alleged to have developed into an informal market of various goods, beyond the intended scope of a mechanism to supply locals with household provisions. The issue of the Sunday market came before the king's justices in 1281, but the bishop's attorney successfully defended that marketing remained within the agreed terms; the court judgement allowed such activity to continue at Ramsbury, so long as it did not become regularized on one particular day of the week.

There was a third complaint from Marlborough in 1275 about unauthorized market competition at Upavon, south-west of Marlborough and even further away than Swindon; again with the same amount of estimated losses alleged. It was charged that the Basset family had for twelve years operated this market, but it had been raised during Henry III's minority. In fact Upavon had markets on Monday and Tuesday, the latter begun during the minority, the former thirteen years prior to the hearing, but both were licensed. So this complaint was of doubtful validity (though pursued fruitlessly, probably in regard to the Tuesday market, in a plea of quo warranto); but the hearing gave Marlborough's burgesses an opportunity to vent their frustrations, as did complaints about long-standing encroachments on the streets and incidents of corruption on the part of the constable of the castle and the county sheriff.

Some of this frustration may have stemmed from the fact that, at some time before the hundredal enquiries of 1255 mercantile agents and tenants of the Bishop of Salisbury, along with those of a number of other religious and secular lords, had begun to refuse to pay tolls due the Marlborough market, presumably on the grounds of chartered exemptions; it was claimed this had reduced the profits of the market by £10. Since the burgesses had been holding their borough at farm from 1224, this loss of revenues affected them directly. Another indication of the ferocity and bitterness of commercial competition developing by this period is seen in the fact that, on the same day in 1247 that Marlborough was granted its fair beginning on the evening of the festival of SS. Peter and Paul, the king issued orders to the sheriff of Berkshire to suppress a commercial event held in the evening of the same festival at the manor of Wittenham, on the grounds it would draw custom away from the fair; such events were in London known as evechepynges. This concern might seem surprising, as Wittenham was over twenty-five miles away from Marlborough, and yet whoever persuaded the king to grant the fair was evidently well aware of potential competitive threats.

The borough remained in royal hands up to the early fifteenth century, administered by the constables of the castle. Its connection to the road system and the pro-active development under Norman lordship brought Marlborough economic success as regards both its commerce and its industry, not to mention encouraging the development of inns along its High Street, to service travellers. King John's patronage and the establishment of several monastic institutions in the thirteenth and early fourteenth centuries both reflect the growing prosperity of the borough and served to aid it. A small Jewish community had established itself there by 1241, providing a source of monetary loans, though was transferred to Devizes during the early reign of Edward I. Evidence from fourteenth-century taxations rank it as the fifth most significant borough in the county. The plentiful sheep of the region and the good local water supply provided the resources to fuel the leather- and cloth-based industries – shown by surname and other evidence to be present in Marlborough; initially artisans engaged in cloth-making were of lower status, allowed to work only for burgesses and not to acquire burgess status themselves. Merchants are also well evidenced: we hear of wine, garlic, fish, woad, grindstones, iron and steel being handled by them, or brought through the borough market, en route to or from Salisbury, Bristol, and particularly Southampton. In 1280 consideration had been given to transferring the county court to Marlborough from Wilton, although this was not effected.

The town's economy, in the later medieval period, was robust enough to deal with the adversities of the fourteenth century and with the declining importance of the castle, the latter perhaps encouraging refocusing of the urban community in the vicinity of St. Mary's, around which further burgage units had developed, in something resembling a grid plan, possibly from the twelfth or thirteenth centuries. There are some indications of shrinkage in population by the sixteenth century – though the demolition of one of the churches may point only to localised decline – and in the following century a fire in the town centre did considerable damage, but was followed by rebuilding. However, until modern times, Marlborough does not seem to have expanded much beyond its medieval limits, with residential growth concentrated mainly along roads leading outwards from the town centre.