John Finn enlisted in the Navy in July 1926 and went directly into three months' training in San Diego, California. There, he was taught to be an infantryman, in the same way recruits in the Army or Marine Corps were taught, except that naval recruits also learned to pull a boat. At the end of basic training, he was chosen to serve another three months in a guard company. He felt it an honor to be in the guard company; he already knew how to live the life of a sailor in the Navy, but those in the guard company had first choice to go to any of the service schools. Finn had his heart set on going to seaman gunner's school, where he would learn how to load and operate the broadside guns, but it was no longer offered. When Aviation General Utilities School at Great Lakes, Illinois was mentioned, Finn naively thought he could learn how to fly, and volunteered. When he got to Great Lakes, they didn't even mention pilot training but Finn admits that Great Lakes was a good school. It taught him aviation in general, and everything he needed to know to be an aviation machinist's mate, from the Theory of Flight to how to walk on a plane. Finn graduated, and was transferred back to San Diego.

John Finn graduated from Aviation General Utilities School [Annotator's Note: at the Great Lakes Naval Training Center in North Chicago, Illinois] and was transferred back to San Diego in December 1926. He did three months mess cooking, which consisted of general maintenance of the mess hall, then was assigned to the wing shop. There he covered airplane wings, tails, ailerons and fuselages with fabric, and doped the covered surfaces. Finn's crew became highly skilled and was really good at the job but Finn was always agitating to transfer to the gunnery department. As a young sailor, Finn was afraid of any and all rated men, but he finally plucked up the courage to apply for a transfer, sat for and passed an examination, went through more training, and was moved to the North Island Naval Air Station gunnery department.

At North Island Naval Air Station John Finn achieved Seaman First Class, then in September 1929 he made Third Class Petty Officer and was transferred to Fleet Air. He was assigned the USS Lexington (CV-2)]. Finally at sea, Finn was on what was known as a Southern Cruise through the Panama Canal and up to Cuba. He was testing the notorious alcohol torpedoes, and found many of them defective. He moved up to Second Class Petty Officer, and went to the Virgin Islands. When Finn learned that the Navy was putting seven 10,000-ton heavy cruisers in commission, he wanted to get on one of them, and go on a shakedown cruise.

John Finn lucked out. He was the right rating and got transferred to the heavy cruiser USS Houston (CA-30), commissioned in Portsmouth Navy Yard in June 1935. The ship left the east coast and headed for the Asiatic Station, sailing from Panama to the Hawaiian Islands. The Houston carried four airplanes, and one of Finn's duties was catapult gunner, a dangerous job. Fifteen years into his Naval career, he was stationed at Kaneohe Bay as Chief Petty Officer in charge of a 35-man ordinance crew.

John Finn says that all of the outfits in Hawaii were on limited alert and knew that something might happen. Previously, in San Diego, everyone had been charged with advanced assignments in the event of a national emergency. Finn was asked if he would accept the commission of Warrant Gunner, which he begrudgingly accepted, all the while thinking there would never be a national emergency. Like the rest of the enlisted men, Finn didn't know anything but scuttlebutt [Annotator's Note: naval slang for rumors]; the terrible thing was that Navy Admiral Kimmel [Annotator's Note: US Navy Rear Admiral Husband E. Kimmel] and Army General Short [Annotator's Note: US Army Major General Walter C. Short], who were in charge in Hawaii, didn't know the attack was coming either even though the United States had supposedly broken the Japanese code. On 7 December [Annotator's Note: 7 December 1941], early in the morning, the Japanese came and rudely awakened them, kicked the living hell out of them, and burned up all of their planes. Practically every ship in Pearl Harbor received some kind of damage, but the worst damage the Japanese did at Pearl was the destruction of men. To Finn it was a great miracle that the United States could repair and improve the old ships and produce new ships so quickly.

John Finn was on the opposite side of the island from Pearl Harbor, at Kaneohe Bay, and he heard terrible explosions, and remembers making the remark that the battle had started. Finn was in bed, but not asleep, when he heard the first planes fly over, and was perplexed to hear single engine planes, on Sunday, firing slow machine guns. Japanese Zeros [Annotator's Note: Mitsubishi A6M fighter aircraft, also known as the Zeke or Zero] and high and low level bombers had already started to strafe Finn's location with incendiary ammunition. The island was under attack, and Finn sprang into action, headed to the hangar, and spotted a Japanese airplane in the rear view mirror of his car at about the halfway point. He watched as that plane made its way to attack his base area. He arrived in less than five minutes, and three planes in the bay were burning, but the hangars were as yet unharmed. Finn's men went to work to set gun mounts on the ground to accommodate machine guns they took from the PBYs [Annotator's Note: Consolidated PBY Catalina flying boat, or seaplane.] As a chief ordnanceman, Finn can describe the particulars of all the weapons that were under his purvey. Finn jumped on a .30 caliber gun and manned it throughout the attack. They defended the base for two and a half hours and lost 19 men. Finn was awarded a Medal of Honor on 15 September 1942 for his response to the Japanese attack on that day.

John Finn stayed on .30 and .50 caliber machine guns throughout the battle, but couldn't be sure if he shot down any Japanese planes that day. His squadron had worked through the night getting ready for another attack, but the Japanese never came back. Finn sustained 21 shrapnel wounds and lived with fragments in his body for years. Finn was awarded the Medal of Honor at Pearl Harbor in 1942. His wife was allowed to attend the ceremonies on the USS Enterprise (CV-6).