Lord Henry Seymour was a younger son of Edward Seymour, 1st Duke of Somerset and Anne Stanhope. Aboard HMS Rainbow he commanded the 'Narrow Seas' Squadron of ships that took part in the Battle of Gravelines against the Spanish Armada in 1588. Controlling the seas off Dunkirk they joined the main British fleet (the 'Western Squadron') near Calais on 6 August, adding about a third to the total size. Married Joan Percy, daughter of Thomas Percy, 7th Earl of Northumberland.