The former Beatle has recorded instrumental parts on one track on the Travis frontman's debut solo LP, tentatively titled 'Wreckorder'.
Fran was so thrilled to have his idol – who is a strict vegetarian - appear on the song he agreed to became a vegetarian as a thank you.
Writing on his blog on website FranHealy.com, the 'Turn' singer said: "My wife Nora and I were sitting and thinking of a thank you present and I suggested going vegetarian. Our son is already one, so it would mean we would all be on the same boat. I met Paul at his show in Berlin and told him what we wanted to do. He was visibly flabbergasted. Three days later the FedEx man delivered three Linda McCartney cook books. I haven't had any longing for meat or fish yet."
Paul's late first wife Linda was also a strict vegetarian and outspoken animal rights campaigner and famously released her own range of frozen vegetarian meals.
Fran, 36, admits he still can't quite believe Paul agreed to appear on the album.
He wrote on his blog: "One of the biggest coups was getting Paul McCartney to play bass on a song.
"I'm not sure what non-McCartney songs he's played bass on but I couldn't think of many. His bassline is always brilliant."
Paul, 67, can play multiple instruments but was predominantly responsible for playing the bass guitar while in The Beatles.