When was the last time Paul McCartney met, face-to-face, with fellow Beatle John Lennon?

Sunday, 25 April, 1976

The day after they considered an appearance on NBC's Saturday Night (later Saturday Night Live), John Lennon and Paul McCartney met for the final time.

Although the pair were on good terms, the encounter was not a success. McCartney turned up unannounced at the Dakota building, where Lennon lived, and was turned away.

"That was a period when Paul just kept turning up at our door with a guitar. I would let him in, but finally I said to him, 'Please call before you come over. It's not 1956, and turning up at the door isn't the same anymore. You know, just give me a ring.' He was upset by that, but I didn't mean it badly. I just meant that I was taking care of a baby all day, and some guy turns up at the door."
John Lennon, 1980
All We Are Saying, David Sheff

McCartney left for Dallas, Texas, where the following day Wings began rehearsals for the US leg of their world tour. Although he stayed in contact with Lennon until the latter's death in 1980, the pair would never again meet face-to-face.

https://www.beatlesbible.com/1976/04/25/john-lennon-paul-mccartney-final-meeting/

In April 1976, Paul McCartney and John Lennon spent one final evening together at John's New York apartment. They watched Saturday Night Live, laughed at a sketch offering the Beatles a measly $3,000 to reunite, and even toyed with the idea of crashing the studio. But they let it slide and stayed in.

The next day, Paul dropped by again unannounced. John, busy with his young son, asked him to call next time, an awkward moment that ended up being their last in-person interaction.

They did stay in touch by phone afterwards, chatting about everyday things like baking and parenting. Paul remembers those conversations fondly, especially after John's tragic death in 1980.
Their last meeting wasn't dramatic, just two old friends sharing a quiet evening. A gentle ending to one of the most legendary creative partnerships in history.

https://www.express.co.uk/entertainment/music/1427540/Lennon-McCartney-last-meeting-Beatles-reunion-Linda-Yoko-Ono-Saturday-Night-Live