[3] Henderson was appointed an acting sub-lieutenant in the Royal Navy in November 1900, confirmed as such in 1902,[5] and posted to HMS Syren in April that year.

[9] He served in World War I as Commander (executive officer) of the battleship HMS Erin in 1914[9] and took part in the Battle of Jutland in 1916.

[10] In 1917, as a commander involved in anti-submarine warfare, he quietly opposed the Admiralty's official position that the volume of merchant shipping was too great to be protected by warships.

[9] As such he played a significant role in all of the major warship procurements for the Royal Navy in the rearmament period running up to the Second World War, in particular the new aircraft carrier, escort and cruiser forces.

In April 1939, he was promoted to Knight Grand Cross of the Order of the Bath (GCB), which, owing to his illness, was accepted by his wife on his behalf at Buckingham Palace.