De Horsey joined the Royal Navy in 1840 and served on the coast of Syria later that year. He received the Naval General Service Medal and bar for his service in Syria as well as a medal for his service in Acre given by the Sultan of the Ottoman Empire. Promoted to Lieutenant in July 1846 and to Commander in June 1853, he was given command of the paddle sloop HMS" Devastation" that same month and of HMS "Victor" from November 1855. Promoted to Captain in September 1857, he commanded HMS "Brisk" from May 1859, HMS "Wolverine" from May 1864, HMS "Aurora" from November 1865 and HMS "Hector" from May 1868. He was Senior Officer on the Lakes of Canada during the Fenian raids; for this he was awarded the Canadian Medal. In 1871 he was made ADC to Queen Victoria. In July 1872, having been promoted to Commodore, he became Senior Naval Officer in Jamaica with his pennant in HMS "Aboukir" and responsible for superintending Jamaica Dockyard. He captured the Spanish slaver "Manuela", and suppressed riots in Jamaica for which he was thanked in Parliament.