The former The Beatles star and Kanye met up for a top secret songwriting session that began with Paul strumming on his guitar as the rap star looked at photos of his wife, Kim Kardashian, on his iPad - but the get together quickly became productive following a chat about sex.
The conversation broke the ice and the two superstars were off and running.
Several months later, West sent McCartney a completed song, which also featured Rihanna.
"I was sitting around, just strumming a little groove... and nobody said, 'Let's make a song of that'," he tells Billboard, "but months later I got a song with Rihanna on it and I said, 'Where am I?'"
"I didn't recognise it because they changed the key."
But Paul is still a big fan of "FourFiveSeconds", adding, "I thought that record was great. Every time we go to a club, my wife Nancy requests it."
He and Kanye also worked on another tune, "Only One", which was inspired by their mothers' deaths.
"We were telling stories, and at one point I told him how 'Let It Be' came from a dream about my mother, who had died years before, where she said, 'Don't worry, just let it be.'"
"He (West) said, 'I'm going to write a song about my mother', so I sat down at this little Wurlitzer keyboard and started playing some chords, and he started singing. I thought, 'Oh, are we going to finish this?' but that was that. And it became 'Only One'."