After Kennedy's military service as commander of the Motor Torpedo Boat PT-109 during World War II in the South Pacific, his aspirations turned political. With the encouragement and grooming of his father, Joseph P. Kennedy, Sr., Kennedy represented Massachusetts's 11th congressional district in the U.S. House of Representatives from 1947 to 1953 as a Democrat, and served in the U.S. Senate from 1953 until 1960. Kennedy defeated then Vice President and Republican candidate Richard Nixon in the 1960 U.S. presidential election, one of the closest in American history. He was the second-youngest President (after Theodore Roosevelt), the first President born in the 20th century, and the youngest elected to the office, at the age of 43. Kennedy is the first and only Catholic and the first Irish American president, and is the only president to have won a Pulitzer Prize. Events during his administration include the Bay of Pigs Invasion, the Cuban Missile Crisis, the building of the Berlin Wall, the Space Race, the African American Civil Rights Movement and early stages of the Vietnam War.

Kennedy was assassinated on November 22, 1963, in Dallas, Texas. Lee Harvey Oswald was charged with the crime but was shot and killed two days later by Jack Ruby before he could be put on trial. The FBI, the Warren Commission, and the House Select Committee on Assassinations concluded that Oswald was the assassin, with the HSCA allowing for the probability of conspiracy based on disputed acoustic evidence. The event proved to be an important moment in U.S. history because of its impact on the nation and the ensuing political repercussions. Today, Kennedy continues to rank highly in public opinion ratings of former U.S. presidents.

John Fitzgerald Kennedy was born at 83 Beals Street in Brookline, Massachusetts on Tuesday, May 29, 1917, at 3:00 p.m., the second son of Joseph P. Kennedy, Sr., and Rose Fitzgerald; Rose, in turn, was the eldest child of John "Honey Fitz" Fitzgerald, a prominent Boston political figure who was the city's mayor and a three-term member of Congress. Kennedy lived in Brookline for the first ten years of his of life. He attended Brookline's public Edward Devotion School from kindergarten through the beginning of 3rd grade, then Noble and Greenough Lower School and its successor, the Dexter School, a private school for boys, through 4th grade. In September 1927, Kennedy moved with his family to a rented 20-room mansion in Riverdale, Bronx, New York City, then two years later moved five miles (8 km) northeast to a 21-room mansion on a six-acre estate in Bronxville, New York, purchased in May 1929. He was a member of Scout Troop 2 at Bronxville from 1929 to 1931 and was to be the first Boy Scout to become President. Kennedy spent summers with his family at their home in Hyannisport, Massachusetts, also purchased in 1929, and Christmas and Easter holidays with his family at their winter home in Palm Beach, Florida, purchased in 1933. In his primary school years, he attended Riverdale Country School, a private school for boys in Riverdale, for 5th through 7th grade.

For 8th grade in September 1930, the 13-year old Kennedy was sent fifty miles away to Canterbury School, a lay Catholic boarding school for boys in New Milford, Connecticut. In late April 1931, he had appendicitis requiring an appendectomy, after which he withdrew from Canterbury and recuperated at home.

In September 1931, Kennedy was sent to The Choate School (now Choate Rosemary Hall), an elite boys boarding school in Wallingford, Connecticut, for his 9th through 12th grade years. His older brother Joe Jr., was already at Choate, two years ahead of him, a football star and leading student in the school. Jack thus spent his first years at Choate in his brother's shadow. He reacted with rebellious behavior that attracted a coterie. Their most notorious stunt was to explode a toilet seat with a powerful firecracker. In the ensuing chapel assembly the autocratic headmaster, George St. John, brandished the toilet seat and spoke of certain "muckers" who would "spit in our sea." The defiant Jack Kennedy took the cue and named his group "The Muckers Club." Kennedy remained close friends to the end of his life with several of his Choate fellows, including especially Kirk LeMoyne "Lem" Billings. Throughout his years at Choate, Kennedy was beset by health problems, culminating in 1934 with his emergency hospitalization at Yale-New Haven Hospital from January until March. In June 1934 he was admitted to the Mayo Clinic in Rochester, Minnesota and diagnosed with colitis. When Kennedy graduated from Choate in June 1935 his superlative in The Brief, the school yearbook (of which he had been business manager), was "Most likely to Succeed."

In September 1935, he sailed on the SS Normandie on his first trip abroad with his parents and his sister Kathleen to London with the intent of studying for a year with Professor Harold Laski at the London School of Economics (LSE) as his elder brother Joe had done. Mystery surrounds his time at LSE and there is uncertainty about how long he spent there before returning to America. In October 1935, Kennedy enrolled late and spent six weeks at Princeton University. He was then hospitalized for two months' observation for possible leukemia at Peter Bent Brigham Hospital in Boston in January and February 1936. He recuperated at the Kennedy winter home in Palm Beach in March and April, spent May and June working as a ranch hand on a 40,000-acre (160 km²) cattle ranch outside Benson, Arizona, and in July and August raced sailboats at the Kennedy summer home in Hyannisport.

In September 1936 he enrolled as a freshman at Harvard College, where he produced that year's annual Freshman Smoker, called by a reviewer "an elaborate entertainment, which included in its cast outstanding personalities of the radio, screen and sports world." He tried out for the football, golf, and swimming teams. He earned a spot on the varsity swim team. He resided in Winthrop House during his sophomore through senior years, again following two years behind his elder brother, Joe. In early July 1937, Kennedy took his convertible, sailed on the SS Washington to France, and spent ten weeks driving with a friend through France, Italy, Germany, Holland, and England. In late June 1938, Kennedy sailed with his father and his brother Joe on the SS Normandie to spend July working with his father, recently appointed U.S. Ambassador to the Court of St. James's by President Roosevelt, at the American embassy in London, and August with his family at a villa near Cannes. From February through September 1939, Kennedy toured Europe, the Soviet Union, the Balkans, and the Middle East to gather background information for his Harvard senior honors thesis. He spent the last ten days of August in Czechoslovakia and Germany before returning to London on September 1, 1939, the day Germany invaded Poland. On September 3, 1939, Kennedy and his family were in attendance at the Strangers Gallery of the House of Commons to hear speeches in support of the United Kingdom's declaration of war on Germany. Kennedy was sent as his father's representative to help with arrangements for American survivors of the SS Athenia, before flying back to the U.S. on Pan Am's Dixie Clipper from Foynes, Ireland to Port Washington, New York on his first transatlantic flight at the end of September.

In 1940, Kennedy completed his thesis, "Appeasement in Munich," about British participation in the Munich Agreement. He initially intended his thesis to be private, but his father encouraged him to publish it as a book. He graduated cum laude from Harvard with a degree in international affairs in June 1940, and his thesis was published in July 1940 as a book entitled Why England Slept, and became a bestseller. From September to December 1940, Kennedy was enrolled and audited classes at the Stanford Graduate School of Business. In early 1941, he helped his father complete the writing of a memoir of his three years as an American ambassador. In May and June 1941, Kennedy traveled throughout South America.

In the spring of 1941, Kennedy volunteered for the U.S. Army, but was rejected, because of his chronic lower back problems. Nevertheless, in September of that year, the U.S. Navy accepted him, because of the influence of the director of the Office of Naval Intelligence (ONI), a former naval attaché to Joseph Kennedy. As an ensign, Kennedy served in the office which supplied bulletins and briefing information for the Secretary of the Navy. It was during this assignment that the attack on Pearl Harbor occurred. He attended the Naval Reserve Officer Training Corps and Motor Torpedo Boat Squadron Training Center before being assigned for duty in Panama and eventually the Pacific theater. He participated in various commands in the Pacific theater and earned the rank of lieutenant, commanding a patrol torpedo (PT) boat.

On August 2, 1943, Kennedy's boat, the PT-109, along with PT-162 and PT-169, were ordered to continue a nighttime patrol near New Georgia in the Solomon Islands when it was rammed by the Japanese destroyer Amagiri. Kennedy was thrown across the deck, injuring his already-troubled back. Nonetheless, Kennedy gathered his men together and swam, towing a badly burned crewman by using a life jacket strap he clenched in his teeth. He towed the wounded man to an island and later to a second island from where his crew was subsequently rescued. For these actions, Kennedy received the Navy and Marine Corps Medal under the following citation: