Erskine was the only son of
Henry Erskine, second son of the second marriage of
John Erskine,
Earl of Mar, and heir to the
Barony of Cardross, by his wife Margaret, only daughter of Sir James Bellenden of Broughton, near Edinburgh. On the death of his grandfather in December 1634 he became vested in the title of Cardross, and was served heir to his father in the barony, 17 March 1636–7. He was one of the few peers who protested against the delivering up of
Charles I to the
English army at Newcastle in 1646, and was a promoter of the "engagement" in 1648, for which he was fined £1,000, and debarred from sitting in parliament in 1649. ==Family==