Derek Taylor, the former press secretary for the Beatles, heard Nilsson's album and sent copies to his friends including the Beatles. When John Lennon and Paul McCartney arrived in New York to announce the formation of Apple, they held a press conference. When asked to name their favorite American singer, they responded "Nilsson." Asked to name their favorite American group, they again said "Nilsson."
At the Beatles's invitation, Nilsson visited the Beatles's in London while the Fab Four recorded their "White Album." One evening, Paul McCartney visited Nilsson and his wife Diane in their hotel room. McCartney played his guitar while singing selections from the album. One of the songs was "Mother Nature's Son" which Nilsson included on his 1969 album Harry.
After Paul McCartney announced his departure from the Beatles, rumors spread that the group would continue with the addition of Harry Nilsson and Billy Preston.
Nilsson became friends with John Lennon and Ringo Starr. After purchasing an apartment in London in the early 1970s, Nilsson hired Ringo Starr and Ringo's furniture-designer partner, Robin Cruikshank, to redecorate the apartment. After inspecting the result, Nilsson asked them to replace a stained-glass panel in the bathroom which included an image of a hangman's noose.
In the mid-1970s, Nilsson and John Lennon's antics during Lennon's "lost weekend" generated newspaper headlines especially after the two were ejected from the Troubadour in Los Angeles for heckling the Smothers Brothers.
Harry Nilsson spent of 1974 working (and partying) with John Lennon and Ringo Starr. Lennon produced Nilsson's Pussy Cats album which was released that year. Nilsson helped Lennon write the song "Old Dirt Road" (Nilsson contributed the line "Trying to shovel smoke with a pitchfork in the wind) and sang backing vocals for the song which was released on Lennon's Walls and Bridges album.
Also in 1974, Nilsson and Ringo Starr appeared in the film "Son of Dracula" with Harry starring as "Count Down" and Ringo playing the Merlin the Magician. After Son of Dracula, Nilsson and Starr worked on a film called "Ringo And Harry's Night Out" which was never completed. Helping Ringo record his Goodnight Vienna album, both John Lennon and Harry Nilsson recorded guide vocals for "Only You." Later someone combined bootleg versions of the two vocal tracks to create a Lennon/Nilsson duet.
On November 28, 1979, the Los Angeles home Ringo Starr rented from Harry Nilsson caught fire. No one was injured, but Starr lost many of his Beatles momentos.
In 1980, after John Lennon was shot and killed outside his New York home, Nilsson traveled to New York to console Lennon's widow, Yoko Ono. Nilsson also offered his home to Ringo Starr who stayed with Nilsson, avoiding the press in the days after Lennon's murder. Nilsson dedicated himself to supporting sensible gun control laws and became the spokesperson for the Coalition to Stop Gun Violence.
When Harry Nilsson died in 1994, George Harrison attended his funeral. Ringo Starr was out of the country and unable to attend, but Starr's wife, Barbara Bach, was there.