He pursued a military career, being present at several battles, including Culloden in 1745, where he was aide-de-camp to the Duke of Cumberland. He rose to the rank of lieutenant general in 1759 but was dismissed in 1764 for opposing what he felt were arbitrary measures put forward by King George III and his ministers in their attempts to suppress John Wilkes and his freedom-of-the- press campaign.
Conway was secretary of state for the Southern Department between July 1765 and May 1766, in the George Grenville administration. Subsequendy, he was secretary of state for the Northern Department between 1765 and 1768, in the administrations of the second Marquess of Rockingham and the first Earl of Chatham, William Pitt. Conway was also lieutenant general of the ordnance between 1767 and 1772.
He was promoted to general in 1772, and to field marshal in 1793. He was also governor of Jersey, where he occasionally resided, from 1775 until his death in 1795. Conway opposed the prosecution of the American war between 1775 and 1781, but nevertheless became commander in chief of the British army in 1782. He held that post until 1784, when he resigned after launching a personal verbal attack on William Pitt, the Younger.