The English Heritage blue plaque is the second to honour Lennon; the first was unveiled in December 2001 at Mendips, Lennon's childhood home at 251 Menlove Avenue, Liverpool.
The plaque at 34 Montagu Square, London, was unveiled by Ono to a crowd of around 100 people. Among them was Rod Davis, the banjo player in Lennon's first group The Quarrymen.
Beatles biographer Hunter Davies opened the ceremony, before Yoko Ono made a brief speech.
I am very honoured to unveil this blue plaque and thank English Heritage for honouring John in this way. This particular flat has many memories for me and is a very interesting part of our history.
In what would have been John's 70th year, I am grateful to you all for commemorating John and this particular part of his London life, one which spawned so much of his great music and great art.
Lennon and Ono borrowed the basement and ground floor maisonette at 34 Montagu Square from Ringo Starr, who had leased the property from 1965. Starr lived there for a brief time, and it was later used by Paul McCartney in 1966 and Jimi Hendrix the following year.
The couple moved in shortly after beginning their relationship. It was at 34 Montagu Square that, in the summer of 1968, they took the controversial cover photography for their first long-player, Unfinished Music No 1: Two Virgins, which featured the pair nude.
On 18 October 1968 the property was raided by the police, who confiscated a small amount of cannabis resin. The pair were arrested, but Lennon took sole responsibility and pleaded guilty on pleaded guilty on 28 November.
He [Lennon] would think it highly amusing that they've put up a plaque here, considering what happened here. If only they'd mentioned the drugs bust on the bottom of the plaque. Wherever he is, he would have a great chuckle about this one.
Foreign tourists are often quite incredulous that more isn't made of the Beatles and Lennon over here.