She was commanded for her entire career by Lieutenant-Commander Malcolm David Wanklyn, and became the most successful British submarine of the Second World War. After a working up period, she left for Malta on 10 December 1940 and was attached to the 10th Submarine Flotilla based there. She completed 24 patrols, sinking 93,031 tons of enemy shipping including four warships; the "Libeccio" after the Battle of the Duisburg Convoy, two submarines (the "Tricheco" and the "Saint Bon") and an auxiliary minesweeper, and ten merchant ships; three troopships, six cargo ships, and an auxiliary transport. Wanklyn was awarded the Victoria Cross for a patrol in her in 1941, which included an attack on a particularly well-defended convoy on 24 May 1941 in which "Upholder" sank the Italian troop ship . On 28 July 1941 she damaged the Italian cruiser "Giuseppe Garibaldi" (9500 tons). On 18 September 1941 she sank two troopships within hours of each other: the sister ships () and ().