Paul McCartney told Mojo magazine October 2008 the story of this song: "With 'Birthday' we had a few friends around and it was one of our party's birthday, can't remember who. Pattie Boyd was there, Terry Dolan, just a few mates. Normally we didn't have friends around to sessions so it was very unusual. We didn't know what song to do so we decided to make one up. We did what Roy Orbison had done with 'Pretty Woman' and Little Richard had started with 'Lucille,' do-do do-do do-do do-do; Roy Orbison goes, do-do do-do DO-DO DO-DO- he just changes the end a little bit. We changed basically the same riff of Lucille and Pretty Woman into Birthday- do-do do-do do-do do-do…'You say it's your birthday.'"

The unique sound of this song was not supplied by an organ or any kind of keyboard. It came from running a guitar through a Leslie speaker. Such speakers are commonly found in keyboard instruments. The speaker rotates, which is what provides the different sound.