Rosslyn joined the Metropolitan Police Service in 1980 on the recommendation of his third cousin Lord Strathnaver, a former detective and heir apparent of The 24th Countess of Sutherland. He reached the rank of chief inspector in the 1990s. In 1994 he led the undercover Operation Troodos, a successful crackdown on drug dealers in west London, including the drug supplier of the Marquess of Blandford, later Duke of Marlborough. From 2003 to 2014, he was head of the Royalty and Diplomatic Protection Department (since amalgamated into Protection Command). He received the Queen's Police Medal in the 2009 New Year Honours and was reputedly the Queen's "favourite policeman".