John Lennon added Ono as a middle name by deed poll in 1969.
The change followed Lennon’s marriage to Yoko Ono on 20 March 1969. On 22 April that year, he legally changed his name from John Winston Lennon to John Ono Lennon, replacing “Winston” with “Ono” as his middle name. The change was a public expression of his partnership with Ono as well as a formal legal act.
The timing placed the name change within a highly visible period for the couple. They had held their Amsterdam bed-in for peace soon after their wedding and would continue combining activism, performance and media events. Lennon also used the new name in connection with the Plastic Ono Band and his peace campaigning.
A common source of confusion is that official documents did not always omit Winston. Lennon was often known as John Ono Lennon, but some records used the fuller form John Winston Ono Lennon. “Peace” is associated with his activism and the couple’s campaigns, while “Sean” is his son’s name; neither was the middle name added by deed poll.