[1][2] In 1724 he attended Westminster School,[1] and he served as a page of honour to the Prince of Wales, later King George II.

[2] Fitzwilliam entered the Army as a cornet in the Royal Horse Guards on 20 April 1732,[3] and was promoted to lieutenant in 1739.

He served on Cumberland's staff in Flanders in 1747, and in 1754 was returned to Parliament on the prince's interest as member for New Windsor.

[1] On 12 November 1755 Fitzwilliam was made colonel of the 2nd (The Queen's Royal) Regiment of Foot,[5] and he was promoted to major-general in June 1759.

[1] On 17 October 1751 Fitzwilliam married Barbara, widow of William Cavendish and daughter of Edward Chandler, Bishop of Durham;[1] they had no children.