Some sources claim that in 1614, Sackville was elected MP for Sussex and was one of the leaders of the popular party but this is disputed in other records of the Addled Parliament. In 1616 he visited Lyons and procured the release of Sir Edward Herbert who had been arrested there. He was made a Knight of the Bath on 3 November 1616 when Charles I was created Prince of Wales. From 1619 he was a leading member of the party of the Virginia Company which supported Sir Edwin Sandys. On 22 July 1620 he sailed as a commander in the forces sent under Sir Horatio Vere to assist Frederick V, the Bohemian king, who was James I's son-in-law. He was present at the Battle of White Mountain on 8 November 1620.