Born on 22 September 1832, he was the only surviving child of his father William Marsden and his first wife.
He was educated at Wimbledon school and King's College, London, and was admitted a licentiate of the Society of Apothecaries in 1853 and M.R.C.S.
[1] Entering the British Army in 1854 as staff assistant surgeon, Marsden served in the Crimean War.
For three months he was in the general hospital at Scutari; early in 1855 he was sent to Sevastopol with the 38th Regiment, and then acted as a surgeon to the ambulance corps until the end of the war, when he received the Crimean and Turkish medals.
[1] On his return to England Marsden was appointed surgeon to the Royal Free Hospital, London (founded by his father), where he was also curator of the museum and general superintendent.