William O'Brien was the son of Murrough O'Brien, 1st Earl of Inchiquin and Elisabeth St. Leger. He served under his father in various military activities in France and Spain but was captured with his father by the Turks in 1660 and imprisoned in Algiers until the English Parliament had paid a ransom of 7,500 dollars.
He was a Privy Councillor from 1671 to 1685. He was Captain General of the Forces at Tangier from 1674 to 1680 and Colonel of the 2nd Tangier Regiment of Foot from 1675 to 1680. He was attainted in his absence by the Irish Parliament of King James II in 1689.
He was the first Governor of Jamaica and Vice-Admiral of the Caribbean Seas from 1690 to 1692. Elizabeth Herbert, widow of Edward Herbert, 3rd Baron Herbert of Chirbury and daughter of George Brydges, 6th Baron Chandos.
