William FitzWilliam (Lord Deputy)

Sir William FitzWilliam (1526–1599) was an English Lord Justice of Ireland and afterwards Lord Deputy of Ireland. In 1587, as Governor of Fotheringhay Castle, he supervised the execution of the death sentence on Mary, Queen of Scots. He was the Member of Parliament for Peterborough and represented County Carlow in the Irish House of Commons. He lived at Gainspark, Essex, and Milton Hall.

FitzWilliam was born at Milton Hall, Northamptonshire, the eldest son of Sir William (d. 1576) and grandson of William Fitzwilliam, Sheriff of London, who had been treasurer and chamberlain to Cardinal Wolsey and purchased Milton Hall in 1506. On his mother's side FitzWilliam was related to the Earl of Bedford, to whom he owed his introduction to King Edward VI.

In 1543, FitzWilliam married Anne (Agnes) Sidney (d.1602), daughter of Sir William Sidney of Penshurst Place. She was the sister of Frances Radclyffe, Countess of Sussex, who founded Sidney Sussex College, Cambridge. Her brother, Sir Henry Sidney, was married to Lady Mary Dudley and they were the parents of Mary Sidney, Sir Philip Sidney and Robert Sidney, 1st Earl of Leicester. Her brothers-in-law included Sir William Dormer, Sir James Harington and Thomas Radclyffe, 3rd Earl of Sussex. The Fitzwilliams were the parents of four children: