Buccleuch was the son of Sir James Scott, Earl of Dalkeith (son of James Scott, 1st Duke of Monmouth and Anne Scott, 1st Duchess of Buccleuch) and Lady Henrietta Hyde, daughter of Laurence Hyde, 1st Earl of Rochester.
Following the death of his father in 1705, he was styled the Earl of Dalkeith, until he succeeded to the dukedom of Buccleuch, 6 February 1732, on the death of his grandmother, who was Duchess of Buccleuch in her own right.