The Fitzwilliams of Milton claimed descent from the ancient Yorkshire family. The first of the line, William Fitzwilliam’s grandfather, a prosperous London merchant who became household treasurer to Wolsey, purchased Milton, three miles from Peterborough, in 1502. He so entailed his lands in Essex and Northamptonshire that his heir had to resort to private Acts to provide jointures for his own wife Anne Sapcote and his son’s wife Anne Sidney. Manors in Essex, including Gains Park, went to Anne Sidney, and Fitzwilliam did not come into Milton until his mother’s death in 1576. By then he had added to the Northamptonshire estate by retaining part of a speculative purchase made in June 1553, but he had parted with the Essex manor of Madelles.3Finch, 100-3, 106; LP Hen. VIII, xviii; CPR, 1553, pp. 302-4; VCH Essex, iv. 267, 289.
Fitzwilliam’s mother was a kinswoman of Sir John Russell, 1st Earl of Bedford, and he is said to have been brought up in the Bedford household and to have been preferred by the earl to his first office, the keepership of the King’s bench prison: he does not appear to have progressed from this to become marshal, although he sought to do so under Elizabeth. Other kinsmen who could have furthered his career were Sir Anthony Cooke, who had married his aunt and whose daughter Mildred married William Cecil, and the Sidney family.4Finch, 105; CSP Dom. 1547-80, p. 506.
Fitzwilliam may, like Bedford, have rallied to Mary in the summer of 1553. He was knighted at her coronation and a month later he and his brother John, styled ‘the Queen’s servants’, had a grant of various offices at Fotheringay. By then he was sitting in his first Parliament, to which he had been returned for Peterborough on 25 Sept. The Fitzwilliams were neighbours and tenants of the dean and chapter of Peterborough, lords of that city since the erection of the see, and Fitzwilliam, who was the bishop’s godson, may have been their own choice, although he and his fellow-Member (Sir) Walter Mildmay, one of whose daughters was later to marry his heir, could also have counted on the support of the high steward, Bedford. Both Members ‘stood for the true religion’, that is, for Protestantism, and his dissent may explain why Fitzwilliam was not to sit again until the next reign. In December 1555 he stood surety for Christopher Perne, whose opposition in the Parliament of that year may have been the occasion of his arrest two days after its dissolution. Despite his friendship with the martyr John Bradford and the escape of another Protestant prisoner from the King’s bench prison, Fitzwilliam remained keeper there, and towards the end of the reign he and his brother received a lease of meadowland at Fotheringay. It was probably his namesake of Windsor who was active in Ireland during these years. He could well have accompanied Sir Thomas Radcliffe, Lord Fitzwalter and later 3rd Earl of Sussex, to Ireland in 1556, for Fitzwalter married his sister-in-law Frances Sidney, but there is no firm evidence of his going there before his appointment in i559 to succeed (Sir) Henry Sidney as vice-treasurer. Thereafter his career was to lie largely in Ireland, where he served two terms as lord deputy, until his death after a long illness on 22 June 1599. His portrait was painted in 1595.5Strype, Eccles. Memorials, iii(1), 224; (2), 181; PCC 25 Ketchyn; Bodl. e Museo 17; APC, v. 214, 275; Writings of John Bradford (Parker Soc.), ii. pp. xxxiv, 135-7; Harl. 2143, f. 4; CPR, 1557-8, pp. 416-17; R. Bagwell, Ireland under the Tudors, ii. passim; Finch, facing p. 108.