He entered the navy as a midshipman on 1 February 1827, became a passed midshipman, 10 June 1833, and was commissioned a lieutenant on 9 February 1839. During the Mexican War he took part in the expeditions against Tabasco, Tampico, and Tuxpan, and was present at the fall of Vera Cruz. For his gallantry in action he was allowed to have command of the schooner , which had been captured at Tampico in November 1846 and was taken into service, but she was poorly equipped and was lost on a reef off Vera Cruz on 16 December 1846. While serving at Tabasco during the Mexican-American War, he was commended for gallantry in action by Commodore Matthew Perry. He shared a shipboard cabin with his later adversary, Raphael Semmes. The two officers served together on , Semmes as the ship's flag lieutenant and Winslow as a division officer. The two, however, never mention this fact in their respective autobiographies.