His father was Thomas FitzWilliam, 1st Viscount FitzWilliam (1581–1650). His mother was Margaret Plunkett, daughter of Oliver Plunkett, 4th Baron Louth; through his mother he was related to the powerful Bagenal family. As a younger son he did not expect to inherit the title or estates; like many young men in his position he read law, studying at Gray's Inn, and then resolved on a military career. With the help of James Butler, 1st Duke of Ormonde he became a Colonel in the French Army and showed both courage and military skill. On the death of his eldest brother Richard he became heir to the title.