He was educated at Westminster and at Christ Church, Oxford. In 1748, his father died, and he became the seventh Viscount Stormont, an Irish title that entitled him to a seat in the House of Lords upon election.
He entered the diplomatic service and was attached to the British embassy in Paris in 1751. He also spent time in various German states and in Poland and Austria in the 1760s, observing firsthand the rule of Frederick the Great, Maria Theresa, and the Emperor Joseph. He became a member of the Privy Council in 1763. In the 1760s he also was appointed lord justice general of Scotland.
However, his main claim to fame is that he became secretary of state for the Northern Department (from 1782, known as secretary of state for foreign affairs) in 1779, holding that post in the ministry of Lord North and resigning with North in March 1782. In 1794 he became president of the Privy Council.
He died on 9 September 1796, and is buried next to the first Earl of Mansfield, in the North Cross, Westminster Abbey.