"Oh man, it got me emotional there, Paul", Corden says as the song comes to an end.
McCartney happily obliged - and even swapped seats with Corden as they harmonized on some of The Beatles' greatest hits, including "Drive My Car" and "Blackbird".
The 76-year-old former Beatle delighted viewers with a tour around his hometown of Liverpool along with an iconic trip to Penny Lane.
Penny Lane is also on the itinerary, where Sir Paul autographs a street sign and they spot the barber shop in the song, nipping in for a chat.
During the drive, Corden told McCartney: "Your music is so full of positivity and joy and a message of love and togetherness, I feel like it's more relevant now today maybe than it's ever been".
In the midst of singing the classic together, Corden's eyes began to well up.
Next they go to 20 Forthlin Road in Allerton in south Liverpool, where Sir Paul lived as a teenager. "I'll never ever forget it. Take a breath and jump in", Corden tweeted on Thurs.
It is now a National Trust property. I felt so bad when it came out in the press - "Foo Fighters had a awful time, they hated the whole fucking thing".
Sir Paul sits down at the piano and plays When I'm 64. "My dad would be in there (the next room) watching telly, we'd be in here, and we'd just written She Loves You, which was a big Beatles hit". (Corden's newest Carpool Karaoke features McCartney.) Grohl talked about being a little kid and learning how to play guitar by listening to Beatles records, and shared that he still gets nervous whenever he jams with McCartney.