"Jack" underwent shakedown training along the New England coast, sailing from New London 26 April 1943 for service in the Pacific. Reaching Pearl Harbor 21 May, the submarine took on supplies and departed on her first offensive war patrol 5 June 1943. Taking part in the submarine offensive against Japan, she patrolled off Honshū. "Jack" came upon a five-ship convoy 26 June, and in a series of five well-executed attacks, sank the 4,000-ton passenger/cargo ship "Toyo Maru" and the 6,000-ton cargo ship "Shozan Maru". In attempting to torpedo a third ship, the submarine was shaken by a bomb dropped by an airplane, but the alert crew corrected her dangerous diving angle and effected repairs.