In 1941, Kennedy joined the navy to fight in World War II. He was eventually given command of a boat called PT-109. When the boat was rammed by a Japanese destroyer, he and his crew were thrown into the water. He was able to swim for four hours, saving himself and a crewman. However, the ordeal aggravated his back. For his valor and bravery he received the Purple Heart and Navy and Marine Corps Medal, and returned at the end of the war as a hero.

After the war, Kennedy worked for a time as a journalist until he decided to run for the House of Representatives. He won in 1947 and was reelected twice. While in Congress he showed himself to be an independent thinker, not always following the Democratic party line. In 1953, Kennedy was elected as a senator from Massachusetts. Critics were upset that he would not stand up to Senator Joe McCarthy who was conducting his witch hunt for communist sympathizers in the government.

Before becoming president, Kennedy won a Pulitzer Prize for his book Profiles in Courage. This book discussed eight senators over the years who were willing to go against public opinion, often to the detriment of their careers, to do what was right. Some questioned at the time how much of the book he truly wrote himself and how much was done by his research assistants. However, both he and his research assistant Theodore Sorensen denied this rumor.