Situated in the Chelmer Valley, Great Dunmow was preceded, it is thought, by a small Roman town along a spur at the crest of the slope above the river and a postulated Roman fort near the river crossing. This town sat on part of Roman Stane Street, running east-west between Colchester and St. Albans, where it was crossed by two roughly north-south roads, one between Sudbury and London, the other connecting Chelmsford and Cambridge. A second, small settlement lay just north-east, closer to the river, at what later became Dunmow's Church End. Although both saw some reoccupation in the Saxon period, it seems that settlement mainly focused around the parish church at Church End, first documented in a will of 1045; post-Conquest its advowson belonged to the Clares and their successors up to the 1330s. Domesday records several manors within Dunmow, their 1086 tenants-in-chief including Geoffrey de Mandeville, Eudo Dapifer, Ralph Baynard, and Richard Fitz-Gilbert; Arnold the priest was the sub-tenant of the land of Fitz-Gilbert. This fragmentation, not only within Dunmow parish but within Great Dunmow itself, and persisting through the Middle Ages, may help explain confusing evidence related to Dunmow's history. The overall population of Great Dunmow was relatively large – the second largest in Dunmow hundred (which also included Thaxted) – but it is assumed the site of the church would mark the principal Domesday nucleus. However, settlement expanded south-westwards towards Stane Street as that came to carry a growing number of commercial travellers; it is also possible that some existing residents were motivated to move because of flooding in the vicinity of the church – a problem that continued to cause problems for the church in the post-medieval period.

This expansion and/or relocation is probably what prompted Richard Fitz-Simon, tenant under the Clare lords, to acquire in 1219 a provisional grant of a Tuesday market for his manor at Dunmow – at least, he was recorded as owing a palfrey as payment for the licence, and the following year he gave 5 marks in lieu of the palfrey. But in late 1224 Earl Gilbert de Clare, owner of an older but unlicensed market at Great Bardfield, some six miles further north on the London-Sudbury road, and whose family also held a manor in Little Dunmow, raised an objection to Dunmow's market on the grounds of detrimental competition. The following year Fitz-Simon defended that, in conjunction with his application for a licence, the sheriff had duly made an enquiry in adjacent hundreds to see if there were any conflict and concluded there should be none with the Bardfield market, which was on Saturdays. We know no more of the case, other than arguments about procedural issues, but we may assume Fitz-Simon's defence was successful, since after Henry III came of age, Fitz-Simon was prompt in renewing Dunmow's market licence (1227), without a change of day, and also obtained grant of an August fair and permission to convert a wooded area into a park at the same time.

That we then find evidence that his son, Simon Fitz-Richard,was indebted to the king in 1256 for the sum of 50 marks owed for market, fair and warren rights is initially surprising. The location of these franchises is not specified but Letters assumed, for purposes of her Gazetteer, that it was Dunmow, for she knew of no other market with which Simon's name was associated. Yet this could hardly have been a renewal of the Dunmow licence, for it had been granted to Fitz-Simon and his heirs. Furthermore, in 1254 the king had issued to John de Berners senior license for a Friday market and April fair at his manor of (but read 'in') Dunmow, so that we might normally assume Fitz-Richard had transferred part or all of his Dunmow holdings to Berners. At the death of Humphrey de Bohun, 6th Earl of Essex, in 1373, his estates included the manors of Berneston and Newton in Dunmow, other property in Dunmow and elsewhere, which had all previously been held by Sir John de Berners; in 1261/62 John de Berners was said to hold the manors of Newton (Ess.) and Waldingfield (Suff.) of Ralph de Berners, who seems to have been the head of the family and (as were his ancestors) tenant of Berneston, while in 1487 Newton was said to belong to the manor of Berneston. Newton, comprising by1419 an area of about 140 acres, apparently lay at the north-west end of the High Street, if we may judge from the location of present-day Newton Green, while Barnston (as now known) was two miles south-east of Dunmow and its former lord is still remembered in the street-name Berners End.

The Bohuns had taken over the Dunmow lands of the Mandevilles, previous holders of the earldom, and were evidently trying to expand their holdings there; in 1324 the widow of a Simon Fitz-Richard complained that her late husband, after their marriage, had in 1320 enfeoffed Humphrey de Bohun, 3rd Earl of Essex, in his manor at Dunmow, whereby she was deprived of dower therein following Humphrey's forfeiture after his death at Boroughbridge (Simon having died about two weeks prior to that battle). It is hard to imagine that markets could have been set up on more than one of the manors in Dunmow without exciting some complaint of unfair competition, though it is conceivable that spread of settlement from Church End had taken place within more than one of the Dunmow manors, so that the informal marketplace in the newly-settled area was under shared lordship – as was the case at Brentwood and Billericay – with each lord licensing a market event on different days but in much the same space. Stane Street may well have served as a boundary between manors.

However, an alternative explanation is available for the 1256 record. That record comprises mandates copied into the Close Roll, directed to the sheriff of Essex concerning the taking and release of distraints on Simon, to force him to provide surety for payment of the 50 marks; the sheriff of Essex was the addressee because Simon had properties in Essex on which distraint could be made. Dunmow may have been the family seat, or just possibly Little Dunmow, which had been granted, after its forfeiture by William Baynard (1111), to Robert Fitz-Richard – husband to the daughter of Simon de Senlis (earl of Huntingdon and Northampton) and a younger son of Richard Fitz-Gilbert, founder of the Clare family; the coincidence of Christian names is interesting, though far from conclusive, for Robert was ancestor of the Fitz-Walter barons, lords of Little Dunmow, who were probably the most influential manorial lords in Dunmow in the fourteenth century. The aforementioned franchises, rather than pertaining to Dunmow, more likely belonged to the manor of Pensthorpe in Norfolk. The 1086 tenant of Pensthorpe, holding of Reynold Fitz-Ivo, was one Ranulf, predecessor and conceivably an ancestor of the Simon Fitz-Richard de Penegisthorp who was tenant of Pensthorpe in the time of Richard I, while another of like name was its lord in 1234 [Charles Parkin, An Essay towards a Topographical History of the County of Norfolk, London, 1807, vol.7 pp.119-20]. In the inquisition post mortem on Gilbert V de Clare (1314) a later Simon Fitz-Richard is seen as feoffee of the Clares in thirteen locations in Norfolk and Essex, including at Dunmow and Pensthorpe; most of these had probably been part of the Giffard inheritance of Richard Fitz-Gilbert's wife. In 1257 one of the Simon Fitz-Richards was sued by the Prior of Cokesford for initiating a market at Pensthorpe to the detriment of the prior's at East Rudham (unlicensed, though fairs had been licensed there in 1227 and 1251). It seems likely that the franchises Simon had purchased in or before 1256 related to Pensthorpe. The dispute was, however, settled by an out-of-court agreement between the parties in 1258, Simon agreeing not to allow markets to take place on Tuesdays or Thursdays, without the prior's consent. In the hundredal enquiry of 1275 Simon Fitz-Richard was said to have the assize of bread and ale, free warren, and a gallows at Pensthorpe, and in 1287 a jury further presented him for having a July fair in the manor; the quo warranto proceedings two years before had, typically, focused on Simon's claim to view of frankpledge and the assize of bread and ale at Dunmow, although also challenging a Gilbert son of William (de Clare?) for the same at Great Dunmow.

The family held Pensthorpe up to the reign of Richard II, and evidently retained landed interests at Dunmow after apparently alienating the market settlement to, or sharing it with, the Berners family. The latter descended from a follower of the Conqueror, accumulated estates in various southern counties, some in Essex and Surrey being held of the Clares; a Ralph de Berners claimed before quo warranto proceedings in 1278/79 that his market and fair at Great Horsley (Surr. now West Horsley), perhaps the family's main base, was held by a grant from Henry III, although a jury noted that neither was in operation, as they had failed to attract traders – perhaps partly because the residents, being exempt from tolls at nearby Guildford, whose market was on an important route connecting London with Winchester and Southampton, found they could do better business there. Despite the Berners' landed possessions, John de Berner had run up debts to London and Norwich Jews sufficiently large that he had to obtain the intercession of Prince Edward to persuade the king to grant (1260) that the debts be paid off at the manageable sum of 30s. a year; John had mortgaged his Dunmow property to cover his debts, and one loan contract had also specified a hefty sum as penalty for default in payment, so the king's concession protected Berner's hold on the Dunmow estate, with its market rights.

That Dunmow's market performed better for the Berners than that at Horsley is indicated by it acquiring the informal alias of Chipping Dunmow before 1345, partly to distinguish it from Little Dunmow, which had by then also become known as Canons Dunmow. Although a Simon Fitz-Richard was identified as the tenant of Dunmow in 1314, holding of the Clares, the farm of Dunmow's market is separately mentioned in the post mortem as being held directly by Gilbert V de Clare, seemingly as an appurtenance of the manor of Great Bardfield; the advowson of Great Dunmow's church is later (1369) seen as an appurtenance of that manor, and after Simon's death (1322) his manor at Dunmow was said to owe service at the manor of Bardfield – this being a reflection of the Clares' policy to integrate all their East Anglian holdings into the administration of the Honour of Clare. The possession by Gilbert V of the farm of Dunmow's market is not easy to explain, given the licences issued to Richard Fitz-Simon and John Berners. Perhaps the family had reacquired Chipping Dunmow, or perhaps the rivalry between the markets at Bardfield and Dunmow had resolved through some out-of-court agreement whereby a Clare had simply bought out the market rights (though a new licence then ought to have been purchased). Nor do we know who was farming, or leasing, the market from the Clares.

Chipping Dunmow is thought to have focused around what later became Great Dunmow's High Street. This may have been a stretch of Stane Street, which connected with Braintree in the east and Bishop's Stortford to the west, the former a market site, the latter with an unlicensed fair (by 1187) and possibly a market; the High Street begins at the point where Stane Street was met by the road from Chelmsford. However, the crescent-shaped High Street appears a brief deviation from the expected route of Stane Street. It has been suggested [by N.P. Wickenden, cited in Matthew Pocock, 37-61 High Street, Great Dunmow, Essex: Archaeological Evaluation by Trial Trenching, unpublished report by Essex County Council Field Archaeology Unit, 2007, p.11] that its line may have been dictated by a possible enclosure around the Roman town; this would assume that early medieval reoccupation avoided the Roman town site. Yet the curving route might also represent a deliberate medieval diversion of the through-road, to draw travellers past a marketplace perhaps established in the twelfth or early thirteenth century. Medieval finds from the Roman town site (mainly, but not exclusively, south and west of the line of the High Street) suggest increasing reoccupation in the Later Middle Ages.

While the High Street itself may have served as marketplace, there is suspicion that the market was held in a widened area on its north-east side, between (and perhaps incorporating) that street and what was once known as Back Lane (now White Street), forming a space that might be perceived as roughly triangular. Cellars along the High Street are concentrated on the opposite side of the High Street, suggesting that this was one initial concentration of market-oriented settlers At roughly the centre of the marketplace was erected a chapel dedicated to St. George; although our earliest documentary reference to it is a papal indulgence, granted in 1456 to help fund a priest to celebrate there when weather was too poor for the journey to the parish church at Church End, remnants of the wooden structure have been dated to the fourteenth century, and an even older origin may be indicated by the fact that the fair licensed by John de Berners was to be held at the festival of St. George. At the north-west end of this posited marketplace arrived a road from the north, connecting to the market town of Saffron Walden (and continuing to Cambridge), and near this junction was the site of a fifteenth-century guildhall, perhaps built for a socio-religious gild, though later used as the Town Hall, while the stocks heard of in 1364 were located nearby. Chipping Dunmow perhaps began as a growth of habitations around the junction of the Chelmsford-Cambridge road with Stane Street, then spread along the future High Street route, with a widened marketplace established around the time of the acquisition of one or other of the market licences in the thirteenth century.

Chipping Dunmow seems to have done well enough in the Late Middle Ages; although the Clares' possession of the market by the early fourteenth century is ambiguous in this regard, the number of survivals of timber-framed buildings from fourteenth and fifteenth centuries provides some testimony to prosperity. One of the Dunmow fairs (probably the summer fair) is mentioned in 1364, because a suspected thief, apprehended at the fair and put in the stocks, had been illegally rescued. A few years earlier, in 1357, there is reference to a Great Dunmow couple acquiring a shop as parcel of a larger property transfer. Thanks probably to leather-processing and cloth industries, the population gradually increased, particularly in the sixteenth century (which is probably when New Street, running south off the High Street, came into being). That a number of drapers, tailors, and a weaver are documented as living in Great Dunmow in the late thirteenth century suggests that although raw cloth production may have been relatively minor, its processing and sale could have been more important to the local economy – such is also true of Hatfield Broad Oak, similarly situated near Stane Street. Dunmow remained one of the Essex cloth centres throughout the Late Middle Ages, the presence of an Augustinian priory at Little Dunmow perhaps providing one steady customer [Michael Gervers, "The textile industry in Essex in the late 12th and 13th centuries: a study based on occupational names in charter sources," Essex Archaeology and History, ser. 3, vol. 20 (1989), p.46]. Surname evidence from other sources, though slight, supports the impression of the presence of tailors and drapers, in contrast to other occupations, which venture little beyond the usual victualling, leather-working, and technical trades typical of large villages; in 1319 we also have reference to a woolmonger at Dunmow.

Yet Chipping Dunmow had no evident burghal characteristics, despite a number of landless messuages having been the subject of property transfers there. Property boundaries in maps of recent centuries give little indication of burgage-type tenements, although the block immediately across the High Street from the posited marketplace warrants closer investigation. Great Dunmow's development from manorial market settlement to town, focused along the High Street and main approach roads to it, appears to have been gradual (essentially organic) over the course of the Late Middle Ages, rather than the result of any obvious planned seigneurial intervention – more thorough research into manorial records and boundaries and a scientific town-plan analysis may throw better light on the situation. By 1555 the community of Great Dunmow had become sufficiently self-conscious and self-confident to obtain a grant of incorporation as 'bailiffs and burgesses' and took over the guildhall beside the marketplace as a base for local administration. The charter confirmed the right to a weekly market and two fairs (neither of which was those licensed by Richard Fitz-Simon or John Berners). Meanwhile the marketplace was gradually being built upon, shops replacing stalls, and the market cross being replaced by a market hall that was probably a less elaborate counterpart of that at Salisbury. Infilling of the marketplace eventually forced the market to relocate north of the original site, where Market Street today is the commencement of the road to Saffron Walden. At what point prior to the charter of incorporation Chipping Dunmow may be said to have attained the character of a town is difficult to say. There are no borough records found earlier than Dunmow's incorporation, and its earlier administration seems purely manorial, although community interests and ambitions likely had found expression though a local socio-religious gild.