Preston is one of several people referred to as the "Fifth Beatle". At one point during the Get Back sessions, John Lennon proposed the idea of having him join the band (to which Paul McCartney countered that it was difficult enough reaching agreements with four). Preston played organ and electric piano for the Beatles during several of the Get Back sessions; some of these sessions appeared in the film Let It Be and on its companion album. Preston also accompanied the band on electric piano for its rooftop concert, the group's final public appearance. In April 1969, their single "Get Back" was credited to "The Beatles with Billy Preston", the only time such a joint credit had been given on an official Beatles-sanctioned release (as distinct from an unsanctioned reissue of some Hamburg-era recordings on which they were the backing group for Tony Sheridan). The credit was bestowed by the Beatles to reflect the extent of Preston's presence on the track; his electric piano is prominent throughout and he plays an extended solo. Preston also worked, in a more limited role, on the Abbey Road album, contributing organ to the tracks "I Want You (She's So Heavy)" and "Something".
Signed to the Beatles' Apple label, in 1969, Preston released the album That's the Way God Planned It, produced by Harrison, the title song from which was a hit single in Britain. His relationship with Harrison continued after the Beatles' break-up in 1970; Preston was the first artist to record Harrison's subsequent international hit "My Sweet Lord", on his 1970 album Encouraging Words, which Harrison co-produced with him. He appeared on several of Harrison's 1970s solo albums, starting with All Things Must Pass; made a notable contribution to the Concert for Bangladesh, the Harrison-organized 1971 charity benefit; performed with the ex-Beatle on his 1974 tour of North America; and played at the 2002 Concert for George tribute, held at London's Royal Albert Hall. Preston also worked on solo releases by Lennon and Ringo Starr.