The foundation of a new town was not a guarantee of its success, but the east coast was a prosperous location attracting international shipping, and Lynn rapidly became one of the most thriving of England's new towns. Under the patronage of the bishop, the young town was doing sufficiently well in regional commerce for merchants to build riverside quays. The diversion of the Great Ouse towards Lynn served to improve its access both to the hinterland and to the sea. Its coastal trade (e.g. fish, pottery, coal) made it one of the most prosperous ports in England during the thirteenth century. At this time the long list of exported goods was dominated by agricultural produce notably wool, grain and salt. Fish, furs, cloth, iron, timber, and especially wine were among its imports; spices reached it from Spain, hawks from Norway. Its trading range covered principally Scandinavia, the Baltic, the Low Countries, the Rhineland, Flanders, and northern France, and some of its own leading families were immigrants from those parts. This trade is also evidenced, indirectly, from the number of Baltoscandinavian beach cobbles incorporated in Lynn's town wall, assumed to have come from ship's ballast. In the following two centuries Lynn's imports increasingly diversified; this was despite its fair having been eclipsed by that of Boston, so that in 1283 the bishop had to try, though unsuccessfully, to salvage the situation by shifting the event from July into August. The royal wardrobe purchased many luxury products at Lynn in that century, and during the Scottish and French wars of the fourteenth century Lynn was an important market for provisioning royal armies. However, its wine trade became increasingly focused on London in, and even before, the fifteenth century.
The Merchant Gild must have had a good deal of influence over the borough's economic policy (insofar as there was any), since the personnel of government and gild overlapped a great deal. Occasionally, in the fifteenth century, the borough assembly held special sessions to discuss mercantile matters and the deliberations were restricted to merchants of the town (although this was not the Merchant Gild per se). Full membership in the gild gave the right to trading with outsider merchants on non-market days as well as wholesaling privileges. It was also possible to buy membership only for the spiritual and socio-religious benefits, which was also open to mothers, wives or widows (Margery Kempe being an example) of members, and even deceased persons. The sons of members were able to join for a much reduced fee.
To support its activities, the Gild had (like that at Ipswich) a monopoly of trade in millstones and possibly on some other types of stone and marble certainly it was active in trade in stones as well as control of the Common Staith, or quay, (rented from the Bishop); the latter brought it modest annual revenues from quayage and cranage. It also acquired numerous properties, from which an annual income in rents was obtained. Further revenue came from entrance fees, the membership including not only local merchants, but also non-resident gentry and merchants from the region who through their membership apparently acquired certain trading rights in Lynn; this was in some regards comparable to the admission of foreign burgesses" at Ipswich. By 1421 the Gild had accumulated a wealth of £1,403, although 86% of this was in the form of debts due it, from loans to its members or the borough. The types of activities or services into which some of this income was fed included:
The borough government's own fiscal resources were well below those of the Merchant Gild's. Its budget during the thirteenth century, when courts and their profits were not in borough hands, was founded upon local taxations mentioned above; they were assessed only on moveables perhaps only on commercial goods owned and typically at the rate of between 3d. and 12d. per £. By the end of the century, from which time taxation records have survived (without any hint that they were an innovation) the tallages were being levied annually and the tallage rolls were being elaborated to the point where they have the appearance of draft financial accounts for the borough: other incomes were being recorded, such as freeman entrance fines, trade licences, and fines for market offences; some assessments due were being assigned to payment of particular borough expenses; and numerous outgoings were recorded as deductions from the assessments of individuals who were owed money by the borough. The tallages might bring in between £70-£90 each, but still do not seem to have sufficed to avoid borough deficits at a time when many expenses were being incurred to assert, defend or expand borough jurisdictions and liberties. This messy approach to book-keeping contributed to popular discontent with the taxation system, which in turn led to the clause of the 1309 composition that tried to limit the frequency and suspected abuses of tallage. Only a couple of examples of tallage rolls are found post-1309, and they are more straightforward lists of assessments only.
Ten years after the composition a very good series of Lynn chamberlains' accounts appears, although an earlier such account (1297/98) appears only like a more organized version of the tallage roll (leading to its possible misclassification in that archival series). Although borough financial management may have been reorganized, local tallages remained the mainstay of the budget; by at least the 1330s they were again being levied annually, and sometimes two in a year amounts raised ranged roughly between £100 to £300 annually, with a single "great tax" in 1346/47 yielding £330. Other sources of income were insignificant by comparison, although mention of rents bringing in £73 in 1338/39 is inexplicable for its uniqueness unless that income was normally separately accounted for. As the century wore on, sources of borough income diversified to include profits from the leet court (beyond the farm paid the Bishop) resulting partly from a "new" leet, gifts and legacies, sale of surplus or confiscated materials or of unredeemed pledges, fines for transgressions against the community (e.g. wandering pigs, failure to answer summonses to attend assembly meetings), leases of lands and butchers stalls, revenues from the half of the Tolbooth (tronage, mensurage and lovecup) farmed from the Black Prince after 1373, and a share from hostage after 1378.
Lynn also faced some very heavy costs in the 1360s and '70s, largely due to the war with France; there were huge expenditures on the building of a ship and a barge for royal expeditions, the town had to contribute towards war aids, and money had to be pumped into upgrading the town defences. This placed an onerous tax burden on townsmen (taxation had been extended beyond burgesses to resident non-burgesses). Perhaps in part because of this, and inspired by the example of the Merchant Gild, from about 1380 onwards fiscal strategy focused on the acquisition of real estate and/or rents as a foundation for a relatively reliable annual income; a large bequest of rents from properties once owned by prominent townsman John Burghard helped set the ball rolling. Community taxation continued to the turn of the century, but was abandoned during a period of popular unrest when borough finances were in disarray. After this, rents and leases of lands, tenements, shops, market stalls, and the town mills were the backbone of borough revenues, which fluctuated around the £100 mark annually; and there were renewed efforts in the 1420s to oblige aliens living in Lynn to buy licences to trade.
In the fourteenth century the limits of agricultural expansion had been reached in East Anglia and there were now a much larger number of market towns and ports competing for trade. Even so, at mid-century Lynn remained among the top dozen towns and among its merchants were some of the leading capitalists of England. In 1373 its advantageous location for waterborne trade between the Midlands and the continent led to its selection as one of the official staple ports through which foreign commerce had to be channelled.
However, its export trade was already being adversely affected by several factors:
Despite decline in its key trade in wool and wine (the latter due to the effects of the Hundred Years War), Lynn was able to weather the economic recession in late fourteenth century, thanks largely to trade with the Baltic through Hanseatic ports (notably cloth exports), which established their own warehouses in Lynn in the early fifteenth century. Wine imports recovered somewhat in the first half of the fifteenth century, during which period Lynn's involvement in the export of cloth exports also peaked, compensating for the negligible wool trade, although London was dominating the cloth trade and grain export remained at least as important to Lynn. None of the exported cloth was produced in Lynn itself; there does not seem to have been any real effort to introduce manufacturing industries into the town, other than to serve local needs. There is no evidence of any residential clusterings of particular crafts that might suggest areas of significant industrial activity, with the possible exception of Damgate in the latter half of the fourteenth century, where we find a number of individuals active in the cloth-making industry, with a fulling mill built in the 1390s near the north-eastern end, while the western end of Damgate connected with Webster Row and Listergate, names both indicative of activities associated with cloth-making. Nor do craft gilds have a very conspicuous role in the medieval history of the town, although we know of a number of socio-religious gilds, some of which appear to have had a craft basis.
In the early fifteenth century, Lynn was trading as far afield as Iceland, although not all the trade was in legitimate goods. Commerce with Scandinavian countries was evidently of much importance to Lynn, and the efforts to capture a share in trade with Iceland jeopardised this. For one thing, it antagonized the Hanse merchants trying to monopolize that trade; in 1415 they complained to the king of Denmark, Sweden and Norway, who persuaded Henry V to prohibit English ventures to Iceland. A prohibition was also in effect when voyages to Iceland are evidenced again in 1429; the borough council feared that breaking this prohibition would lead the Danish king to arrest the merchandize of Lynn merchants trading (only) with the Baltic countries. This in fact occurred, and Lynn responded both by a retaliation against Danes in the town and by sending ambassadors to treat with the king of Denmark. The situation was exacerbated by a complaint of the Bishop of Iceland to the king that Lynn merchants were involved in trading Icelandic children into slavery.
Lynn had long outgrown its expansion phase, but was able to maintain itself. This is reflected in the continued rebuilding of houses, enlargement of churches, and repairs to town wall and gates during the Late Middle Ages, and the building of new halls for the two leading merchant gilds, begun in 1406 and 1424. However, the reduction of the number of constabularies at some point around the late fourteenth or early fifteenth century may reflect depopulation in some part of the town (perhaps around St. James). Despite another economic slump in the latter half of the fifteenth century, Lynn was able to adapt to changing circumstances better than nearby Boston, and it was not until the post-medieval period that Lynn's important role in commerce really declined.