Ernle was the eldest surviving son of John Ernle of Whetham House, Calne and his wife Philadelphia Hopton, daughter of Sir Arthur Hopton of Witham Friary, Somerset. In 1654, he was elected Member of Parliament for Wiltshire in the First Protectorate Parliament. He was elected MP for Wiltshire again in 1660 for the Convention Parliament. In 1661 he was elected MP for Cricklade in the Cavalier Parliament. He was knighted by 4 April 1663. In 1671, he was commissioner for accounts of the commission for loyal and indigent officers and was Controller of Storekeepers Accounts from 1671 to 1680.