Claiborne was an Anglican, a Puritan sympathizer, and deeply resentful of the Calverts' Catholicism. He was one of the signers, along with Virginia Governor John Pott, Samuel Matthews, and Roger Smyth, of a letter to the King's Privy Council, dated 30 November 1629, complaining that Lord Baltimore refused to take the Oath of Allegiance and Supremacy to the Church of England. He sided with Parliament during the English Civil War of 1642–1651 and was appointed to a commission charged with subduing and managing the Province of Virginia and Province of Maryland, both British colonies at the time. He played a role in the submission of Virginia to parliamentary rule in this period. Following the restoration of the English monarchy in 1660, he retired from involvement in the politics of the Virginia colony. He died around 1677 at his plantation, Romancoke, on Virginia's Pamunkey River. According to historian Robert Brenner, "William Claiborne may have been the most consistently influential politician in Virginia throughout the whole of the pre-Restoration period".