Ridden by Paul Townend, the 100/30 favourite produced a scintillating leap at the final fence and held on gamely in the closing stages to deny the gallant Santini (5/1) to register a neck success in the extended three and a quarter-mile event.
Al Boum Photo also became the first horse since Kauto Star in 2007 and 2009 to win the race twice. He enjoyed the same preparation for this year's race as when he captured the 2019 Magners Cheltenham Gold Cup having won his only other start this in the Grade 3 Savills Chase at Tramore on New Year's Day.
Mullins, registering his seventh victory of the week at The Festival™ presented by Magners 2020 and his 72nd victory overall at the meeting said: "We're delighted to win the race for the second year running with Al Boum Photo.
"We just decided not to break winning formula and just take in the same preparation as he did last year. Don't break a winning formula when it works and it's paid off thankfully.
"I thought this year's Gold Cup wasn't anything like last year's. I was just thinking if he had a clear round and a bit of luck then it was all to play for.
"I was happy throughout and Paul was brave on him at the last. Paul's plan came together and I'm delighted for him as well as Marie and Joe Donnelly, so I am just delighted.
"It's fantastic and an absolutely unbelievable day. Paul does his own thing and he probably rides better under a little bit of pressure."
Townend said: "It's amazing. I'd want some boot in the rear end if I had been in front that early and got beat by Santini. I thought that I would never feel anything like the feeling I had here last year, but I think this is even better. It's unreal. It's a credit to Willie - to go to Tramore two years in a row and come here with one run - the man is a genius.
"To be honest, I just went out with the plan of getting into a rhythm like last year. We've missed a few fences but he's always found his feet, and it just happened again at the top of the hill, he was jumping a bit better and I ended up in front a bit sooner than I wanted, but I was able to hang on there and produce him when I needed him at the last. He's a tough animal.
"It was a very different race to last year - we knew that going out there, everyone knew it was going to be a different race to last year, but it's the same result, which is the main thing!
"I didn't have to fight for it; a couple of good jumps got it for me there, and I think that was very important. He has everything, this horse. Guts as much as everything else."