Born the son of the Reverend Richard Seymour and Frances Seymour (née Smith), Seymour was educated at Radley College and Eastman's Royal Naval Academy, Southsea and joined the Royal Navy at Portsmouth in 1852. He was appointed to the corvette and, having been promoted to midshipman, then transferred to the paddle frigate in 1853. He served in HMS "Terrible" in the Black Sea throughout the Crimean War. He was appointed to the second-rate , flagship of his uncle Sir Michael Seymour, Commander-in-Chief, China Station in 1857 and took part in the sinking of the war-junks in June 1857, the Battle of Canton in December 1857 and the Battle of Taku Forts in May 1858 during the Second Opium War.