One of the most important Wilton burgesses of the period, Harleston represented the borough in Parliament on at least 11 occasions. Nothing is known about his origins or early trading activities, but at Michaelmas 1410 he was made steward of the Wilton guild merchant, as such being responsible for the community’s finances. In February 1411, during his term of office, he stood surety for John Tugge and his wife, who were being sued for trespass in Wiltshire. In 1413 he was acting as an executor for a fellow burgess, William Chitterne, and two years later he shared a grant of property in Wilton with John Whithorne (his fellow Member in eight Parliaments between 1414 and 1423), John Hardy and other local men. On 13 May that year he received a recognizance from a Derbyshire esquire, John Fynderne, the penalty for defeasance of which was £40.1CCR, 1409-13, p. 196; 1413-19, p. 275; guild steward’s accts. 1412-13; Wilton deed 219.
It was while attending Parliament that, in November 1415, Harleston obtained a seven-year lease of the subsidy and alnage payable on cloth for sale in Salisbury and elsewhere in Wiltshire, at an annual farm of £80. The lease did not, however, run its full term, but came to an end in 1421, when Harleston acted as a mainpernor for his successor, Robert Ismell† of Devizes. Meanwhile, on 11 Dec. 1417, again during a Parliament, he went bail in £50 for Robert Peny of South Tidworth, Hampshire, who had been committed to the Fleet following trial for dispossessing the influential Sir Thomas Skelton of land; he guaranteed that Peny would appear before the council of the Regent. In 1419 and 1420 he attended the elections, held at Wilton, of the knights of the shire.2CFR, xiv. 111, 394; RP, iv. 111; C219/12/3, 4.
Little else is known of Harleston directly, but in 1422 Nicholas Melbury of Stratford sub Castle, near Salisbury, draper alias merchant, was pardoned his outlawry for non-appearance when summoned to answer him in a plea of debt for £6; and in 1427 Nicholas Morys, a merchant from Cheddar, Somerset, received a similar pardon, in relation to a debt of £2.3CPR, 1416-22, p. 434; 1422-9, p. 365.