During his career, he won seven Grand Slam singles titles—three at Wimbledon and four at the US Open. He is well remembered for his shot-making artistry and the best volley in the game, for his classic encounters with Björn Borg, for his fiery on-court temperament (which frequently got him into trouble), and for the catchphrase "You cannot be serious!" He was inducted into the International Tennis Hall of Fame in Newport, Rhode Island, in 1999. He is considered to be one of the greatest players of all time.

McEnroe was born in the U.S military base at Wiesbaden, where his father was stationed with the United States Air Force. He is of Irish-Jewish descent. When he was less than a year old, his family moved to New York City, and he grew up in Douglaston, Queens.

McEnroe took the tennis world by storm as an 18-year-old in 1977, when he made it through the qualifying tournament into the main draw at Wimbledon, where he lost in four sets to Jimmy Connors in the semifinals. It was the best performance by a qualifier at a Grand Slam tournament, and a record performance for an amateur in the open era.

Shortly after, McEnroe entered Stanford University and won the NCAA singles and team titles in 1978. After that, he joined the professional tour.

McEnroe signed one of the first professional endorsement deals in tennis with Nike in 1978.

McEnroe's game combined shot-making artistry, deft volleys, and a fast, attacking style of play. His sharp reflexes enabled him to return the biggest serves and passing shots masterfully, and the variety, delicacy and quickness of his play delighted crowds. But McEnroe also quickly became known for his competitive fire and volatile temper. Verbal outbursts seemed to be a key way in which he motivated himself to battle through tough situations during matches, but this frequently got him into trouble.

McEnroe won his first Grand Slam singles title in 1979 at the US Open. He defeated his good friend Vitas Gerulaitis in straight sets in the final to become the youngest winner of the championships since Pancho Gonzales, who was also 20, in 1948 (Pete Sampras eventually became the youngest US Open Champion at 19 years old). He won 10 singles and 17 doubles titles that year (for a total of 27 titles, which marked an open-era record).