Following Manfred Mann's split in 1969, Klaus Voormann became a session musician. He performed at the debut live performance by the Plastic Ono Band in Toronto on 13 September 1969, alongside John Lennon, Yoko Ono, Eric Clapton and Alan White.
We didn't have a band then – we didn't even have a group that had played with us for more than half a minute. I called Eric – I couldn't find him, I don't know where the hell he was but I finally got through to him – and I got Klaus, and we got Alan White 'cause we'd cut Instant Karma! round that period so I'd met him, he'd been on that. And I said, look, there's this think on in Toronto, you want to come? They said, 'OK'.
Now we didn't know what to play 'cause we'd never played together before. And on the airplane we're running through these oldies. So the rehearsal for that record, which turned into not a bad record, was on the plane with electric guitars – not even acoustic, you couldn't hear – saying 'Are we doing the Elvis version of Blue Suede Shoes or the Carl Perkins?', you know, with the different break at the beginning.
Voormann moved to Los Angeles in 1971 with his second wife Christine and their son Otto. He performed at George Harrison's Concert For Bangladesh, for which he later won a second Grammy Award. He was introduced onstage at the concert by Harrison with the words: "There's somebody on bass who many people have heard about, but they've never actually seen him, Klaus Voormann."
Session work on Lennon's Plastic Ono Band and Imagine albums followed, as well as Ringo Starr's 1973 album Ringo and Ono's 1970 release Yoko Ono/Plastic Ono Band.
John phoned me one morning in January  and said, 'I've written this tune and I'm going to record it tonight and have it pressed up and out tomorrow – that's the whole point. Instant Karma!, you know. So I was in. I said, 'OK, I'll see you in town.' I was in town with Phil Spector and I said to Phil, 'Why don't you come to the session?'
There were just four people: John played piano, I played acoustic guitar, there was Klaus Voormann on bass and Alan White on drums. We recorded the song and brought it out that week, mixed – instantly – by Phil Spector.
In 1973 he moved to Los Angeles at the urging of Lennon and Starr. There he worked as a session musician for a range of musicians; solo Beatle recordings featuring Voormann include Lennon's Sometime In New York City, Walls And Bridges and Rock 'N' Roll, Harrison's All Things Must Pass, Living In The Material World and Extra Texture, and Starr's Sentimental Journey and Goodnight Vienna.
He played bass, and occasionally saxophone, guitar and piano, on recordings by a range of artists including Lou Reed, BB King, Jerry Lee Lewis, Carly Simon, Randy Newman, James Taylor, Peter Frampton, Howlin' Wolf, Martha Reeves, The Band, Ry Cooder and Harry Nilsson.
Voormann returned to Germany in 1979, where he produced three studio albums and a live release by the group Trio, along with their worldwide hit Da Da Da. Following their split in 1986 he produced and played bass on the debut solo album by Trio singer Stephan Remmler, and a single by the group's former drummer Peter Behrens.
George Harrison's 1988 single When We Was Fab incorporated Voormann's Revolver artwork, along with an updated drawing of Harrison by the artist. Voormann and Harrison remained close following The Beatles' split; he played bass at the Concert For George on 29 November 2002, performing on the song All Things Must Pass.
Within You Without You came about after I had spent a bit of time in India and fallen under the spell of the country and its music. I had brought back a lot of instruments. It was written at Klaus Voormann's house in Hampstead after dinner one night. The song came to me when I was playing a pedal harmonium.
In 1989 Voormann largely retired from the music business to devote more time to his family and to return to graphic design. The following year he began working with Astrid Kirchherr on a book, Hamburg Days, which compiled their memories of the city from 1960-62.
Neil Aspinall of Apple Records commissioned him to design the artwork for The Beatles' Anthology project, which was launched in 1995. The design adorned the Anthology book, the CD collections, and subsequent video and DVD releases.
Working with fellow artist Alfons Kiefer, Voormann painted a depiction of Beatles posters and photographs depicting their progress from Liverpool to Let It Be. The Anthology artwork took the two artists more than 1,000 hours to complete.
In October 2003, Voormann published an autobiography, Warum spielst du Imagine nicht auf dem weißen Klavier, John? Erinnerungen an die Beatles und viele andere Freunde (Why Don't You Play Imagine on the White Piano, John?: Memories of the Beatles and Many Other Friends). It focused mainly on events from the 1960s and 1970s, documenting his times with The Beatles and other musicians, as well as his personal life.
Today Klaus Voormann lives with his family near Munich. His recent projects include Remember Revolver, a book containing illustrations for all 14 songs on the Revolver album.