For many people who had grown up with the Beatles in the 1960s, Lennon's death brought an era crashing to an end.
He was shot four times in the back as he got out of a limousine with his wife Yoko Ono outside their New York City apartment on 8 December 1980.
The Beatles frontman, peace campaigner, and all-round iconoclast, who would have been 60 on 9 October, has since attained legendary status.
Lennon's songwriting partnership with Paul McCartney propelled the Liverpool-based pop group to international stardom and unparalleled commercial success.
For many people who had grown up with the Beatles in the 1960s, Lennon's death brought an era crashing to an end.
"Lennon was a great influence on many people at a formative time in their lives" says Bill Harry, author of The John Lennon Encyclopaedia.
Chapman, 45, who expresses remorse for the murder, said in a 1980 interview: "It was an end of innocence for that time. And I regret being the one that ended it."
Peace
Lennon's murder was all the more shocking for his high-profile advocacy for the peace movement in the late 1960s.
"Lennon's work fighting for peace and an end to the Vietnam War struck a chord with a whole generation," says Harry.
Considered naive and idealistic by the media, Lennon and wife Ono, a Japanese avant-garde artist, devoted their combined talents to campaigning for world peace.
In 1969 they staged their famous "bed-in for peace" publicity stunts; they spent a week in bed at hotels in Amsterdam and Montreal, receiving the press.
Solo career
Lennon is best remembered now for songs like Imagine and Give Peace a Chance, but he had a troubled start to his solo career, writing about heroin withdrawal in Cold Turkey.
His first solo album, John Lennon - Plastic Ono Band, came out of "primal scream" psychotherapy sessions and poured out deeply personal childhood and adolescent bitterness.
The hugely successful album Imagine was followed by an uneven spell, and after a 5-year break from recording, led to dip in his public profile by 1980.
He next planned to come back to Britain to relaunch his career following the release of a long-awaited new album, Double Fantasy.
Many fans disliked the Ono influence, according to Harry.
"People saw the John-Yoko thing as a bit crazy. She wouldn't let him get on with his own thing," he recalls.
Friends who heard his latest material found it disappointing, according to Harry, because the Ono-inspired songs weren't very good.
Keith Badman, author of The Beatles Off The Record, disagrees: "He showed that his music could still cut it."
Rock martyr
After his death biographies elevated Lennon's contribution at the expense of Paul McCartney, according to Harry, who feels "Paul was equally if not more of a genius than John".
Badman agrees that Lennon's contribution can be exaggerated.
"If you didn't have all four of them you wouldn't have had the same music," he says.
But fans are always eager to reappraise pop stars once they have attained rock martyr status.
Michael F Nozzolio, chairman of the US Senate Crime Committee, summed up Lennon's legacy when he wrote to parole board chairman Brion Travis urging them not to release Chapman.
"John Lennon represented a vision of hope, peace and love," he wrote.
"Tragically, his positive message and his life were fatally ended by Mark David Chapman."