After intensive shakedown training operations out of New London, the SS 213 departed March 7, 1942, for combat operations in the Pacific. She transited the Panama Canal, arriving in Pearl Harbor April 3, 1942, and sailed April 20, 1942, for her maiden war patrol. GREENLING drew her first blood in the war in approaches to Truk, with the sinking by a single torpedo of a 3,262-ton armed cargo ship. In this first war patrol, GREENLING set an example of daring aggressiveness and tenacity that did not go un-rewarded. It became the hallmark of her gallant crew that made her one of the most valuable units of the Pacific Submarine Force. After fifty-seven days at sea, she returned to Pearl Harbor June 16, 1942.
During GREENLING's second, third, and fourth war patrols, she continued her achievements. GREENLING saw action in the waters off Truk, participating in a chain of U. S. submarine war patrols off of Honshu, Japan, and patrolling the Tokyo-Aleutian shipping lanes. GREENLING continued to wreak havoc upon the Japanese merchant marine into the beginning of 1943.
Lieutenant Commander Bruton brought GREENLING into Brisbane, Australia, from her fourth war patrol January 31, 1943. By this time, GREENLING had sunk 11 ships totaling 54,557 tons. For brilliant exploits during her first three war patrols, she was awarded the Presidential Unit Citation. The holder of three Navy Crosses, two Legions of Merit and a Commendation Ribbon, Lieutenant Commander Bruton (later promoted to Rear Admiral) was succeeded in command of GREENLING by Lieutenant Commander lames D. Grant, USN.