He was eldest son of George Dalrymple of Dalmahoy, fifth son of John Dalrymple, 1st Earl of Stair, and a baron of the court of exchequer of Scotland, by his wife Euphame, eldest daughter of Sir Andrew Myrton of Gogar.
He passed advocate of the Scottish bar in 1741, but he entered the army and attained the rank of captain.
He was a favourite with his uncle John Dalrymple, 2nd Earl of Stair, who having in 1707 obtained a new charter containing, in default of male issue, a reversionary clause in favour of any one of the male descendants of the first Viscount Stair whom he should nominate, selected him to succeed him in the states and honours on his own death.
He was chosen a Scottish representative peer in 1771, and in the House of Lords opposed the measures which led to the American War of Independence.
They included:[1] By his wife, a daughter of George Middleton, banker, London, Dalrymple had one son John, who succeeded him as sixth earl.