Mansel was a Welshman, the son of Sir Edward Mansel of Penrice and Margam (died 1585), although he later established himself among the gentry of Norfolk. His early naval career is not recorded, but he served in the 1596 raid on Cádiz under Robert Devereux, Earl of Essex, commanding HMS "Vanguard", and was knighted for his part in it. He subsequently took part in Essex's Islands Voyage to the Azores (1597), then held commands off the Irish coast during Essex's campaign in Ireland. In October 1602 he was fitted out with a fleet and with the Dutch helped defeat six Spanish galleys under Federico Spinola at the Battle of the Narrow Seas. As a result, Mansell was named Vice-Admiral of the Narrow seas in 1603 and became Treasurer of the Navy in 1604.