Morikawa, who won the 2020 US PGA Championship on just his second major start, held his nerve superbly on the closing stretch to become the first player to win two different majors at their first attempt.
A closing 66 gave the 24-year-old a winning total of 15 under par and two-shot victory, with Spieth also returning a 66 and world No 1 Jon Rahm doing likewise to share third on 11 under with 54-hole leader Louis Oosthuizen.
“I think when you make history it’s hard to grasp and it’s hard to really take it in,” Morikawa said. “At 24 years old, it’s so hard to look back at the two short years that I have been a pro and see what I’ve done because I want more.
“I enjoy these moments and I love it, and I want to teach myself to embrace it a little more, maybe spend a few extra days and sit back and drink out of this (the Claret Jug).
“When you’re in these moments and you truly love what you do, which I love playing golf and competing against these guys, these are the best moments ever because the nerves push you to just be a better person.
When Morikawa recorded his breakthrough major win at the US PGA at Harding Park in San Francisco it was in front of no fans as Covid-19 restrictions were still in full force.
So to do it in front of full grandstands – 30,000 people were allowed into Royal St George’s every day this week – was a delight for the American and he was also able to use that fact to answer those who questioned his ability to perform under pressure.
“The nerves are definitely up there, but you channel these nerves into excitement and energy, and that puts you away from like a fear factor into ‘This is something I want’,” he added.
“I had nothing to prove. It’s just to you guys (the media). I’m just answering it for you guys that we can play in front of fans.
“They are some of the best fans I’ve ever seen. They truly understand the game. They appreciate the game. We owe them a huge thank you.”
Morikawa began the day a shot behind Oosthuizen, but, with the South African making a stuttering start, Morikawa followed six straight pars with birdies on the seventh, eighth and ninth to open up a three-shot lead over Spieth.
The former Open champion had fallen four behind with two early bogeys, but eagled the seventh and birdied the ninth to be out in 34 and emerge as Morikawa’s biggest rival.
The gap remained three as Morikawa and Spieth traded birdies on the ninth and 10th respectively, before a brilliant par save from over the 10th green kept Morikawa in command.
Birdies from close range on the 13th and 14th saw Spieth cut his deficit to a single stroke, only for Morikawa to also take advantage of the par-five 14th – the easiest hole on the course over the week – to edge two in front once more.
A brilliant up and down from left of the 15th gave Morikawa some vital breathing space and with Spieth unable to birdie any of the last three holes, he could enjoy completing the formalities of an impressive win.
Oosthuizen had been hoping to become the first wire-to-wire winner of the Open since Rory McIlroy in 2014, 11 years to the day since he lifted the Claret Jug at St Andrews.
But after dropping just his fourth shot of the week after missing the green with his approach to the fourth, Oosthuizen thinned his bunker shot on the par-five seventh into more sand to run up a costly six.
Oosthuizen almost made a spectacular hole-in-one when his tee shot on the difficult 11th clattered into the pin, but a bogey on the 13th effectively ended his chances and meant he has finished second, second and third in the last three majors.
Four-time major winner Brooks Koepka had set the early clubhouse target on eight under following a 65, with Scotland’s Robert MacIntyre the leading British player on seven under after a 67.
Defending champion Shane Lowry finished in a tie for 12th as Olympic team-mate Rory McIlroy cut a disconsolate figure after a 71 containing four birdies, three bogeys and a double bogey on the 10th.
The 32-year-old, who has now gone seven years since claiming his last major title at Royal Liverpool, said: “There is a lot of birdies in there but there are a lot of bogeys too.
“I’ve made 17 birdies this week, which is more than enough to challenge to win this golf tournament.
“It’s just I make too many mistakes, and that’s the part that I need to try to get right.
“There is enough good stuff in there to contend at these golf tournaments, but I’m just not allowing myself to do that with some the mistakes I’m making.”