A native of Dartmouth, where his elder brother, John, was bailiff in 1394-5, William Mountfort married the heiress of a burgess of Weymouth, thereby acquiring property there and in Melcombe, Smallmouth and Wyke (Dorset), including rights to a moiety of the profits of the ferry plying between the last two places. In September 1395 his brother demised to him half of the sizeable messuage in Dartmouth where their father had lived, but William chose instead to settle in Bridport. He was admitted to the freedom of the borough at Michaelmas 1399, on paying a fee of £2.
Mountfort’s trading interests extended over a wide area, bringing him into contact with merchants from as far away as York, but would seem to have been mainly concentrated on the West Country. He brought several actions at law in the local and central courts against his many debtors, and bonds have survived for payments to him of various sums amounting to well over £500.
Mountfort attested the returns for the parliamentary elections of all the Dorset boroughs in 1407, and he was one of the committee of four burgesses sent to report in the county court the outcome of the Bridport elections to the Parliaments of 1414, 1417, 1420, 1421, 1425, 1426, 1427, 1429, 1431, 1432 and 1437. In the meantime he had himself attended the shire elections of 1425. A fellow Member in his own first Parliament (1413) was his wife’s stepfather, John Corp of Dartmouth, and on 8 June, while the Commons were still in session, he provided securities in Chancery for Walter Reynell, a former knight of the shire for Devon. In 1422 Mountfort was associated with Sir Humphrey Stafford II in a deed of assignment by a Bridport man of all his effects. Two years later he was acting as a trustee of property belonging to Gervase Jakman of Dartmouth, who was then in the Holy Land.
Mountfort’s profitable trading ventures enabled him to purchase ‘Mortesheigh’, plots for growing hemp and messuages in ‘Stake Lane’, East Street and South Street, Bridport; and in the local cofferers’ account of about 1425 occurs a payment for wine drunk ‘apud Monfortys’ when the common seal of the borough was used.
Mountfort and his wife were members of the fraternity of St. Katherine in St. Mary’s church, Bridport, and it was there that, at the altar of All Saints, they founded a chantry. In 1432, for this purpose, Mountfort entrusted a large part of his estates to feoffees, including John Waryn, canon of Exeter, John Jaybien of Plymouth and William Oliver of Kingsbridge, a kinsman of his, with reversion to Oliver’s heirs. By his will made on 28 Apr. 1437 he completed arrangements for his interment in a newly constructed tomb in St. Mary’s. He died before 12 June having left the residue of his lands and goods to his widow, who survived him for at least seven years, and to William Oliver.