He fought on the Royalist side during the English Civil War, but later made his peace with the Cromwellian regime.
They had a record of loyalty to the English Crown, and the viscountcy was conferred on the elder Thomas in 1629 by Charles I in recognition of his good services to the Royal cause.
[2] As a young man William appears to have lived very much in the shadow of his elder brother Oliver FitzWilliam, 1st Earl of Tyrconnell.
Both gained the trust of Charles I of England, and while Oliver returned to fight in Ireland, William became Governor of Whitchurch and Lieutenant General for Shropshire.
He died at his townhouse in the parish of St. Nicholas Within, near Christ Church Cathedral,[2] whose bells at his request were rung on his death in 1674.