At just 24 years old, Morikawa claimed his second major thanks to a round of 66 without ghosts to finish at 15 under par.
The American hadn’t even played in link condition until last week’s Scottish Open, when he tied for 71st, the worst result of his professional career.
But the world number four showed no signs of inexperience as he reveled in the sweltering conditions as temperatures reached 30 degrees Celsius in Sandwich, on England’s southeast coast.
Leader overnight Louis OosthuizenThe bogey in the fourth allowed Morikawa to take part in the lead and he never looked back after a two-shot swing at par-five of the seventh.
A dream debut 👏Congratulations, Collin Morikawa, Golfer of the Year 🏆 #TheOpen https://t.co/x1TfIPewYI
– The Open (@TheOpen) 1626629598000
Oosthuizen, who finished 11-under in a portion of the third with Jon Rahm, twice found bunkers around the green for another shot.
Meanwhile, Morikawa drained the first of three consecutive birdie putts to hit the turn to 14 low.
The 2020 US PGA Champion needed his putter to stay warm in the final nine seconds just to save par at 10, 11, and fundamentally 15 with Spieth breathing down his neck.
But at the time, he had moved to 15 low with another long-range putt at par five 14.
Three solid home pairs made Morikawa the first British Open rookie to win since Ben Curtis’ surprising success also at Royal St. George’s 18 years ago.
Spieth regretted his sloppy finish in the third round on Saturday when bogeys on the final two holes saw him start day two behind Morikawa.
The 2017 British Open champion fell back to minus seven after dropping another two shots in the fourth and sixth before coming back in a way that suggests he won’t wait long to add his top three.
An eagle in the seventh launched Spieth’s challenge and completed the final 12 holes at six under par with more birdies in the ninth, 10th, 13 and 14 to put pressure on Morikawa.
But Morikawa never flinched to become the first man to win two majors in his debut at that event.
Rahm also showed why he was the pre-tournament favorite after landing his first major at the US Open last month with a brilliant 66 that could have been much better but for a better putter day.
The Spaniard needed an eagle in the seventh to get him going, but he ran out of holes after four consecutive birdies between 13 and 16.
Oosthuizen’s wait for a second major continues as he finished in the top three for the eighth time since his only success at the 2010 British Open in St Andrews.
The impeccable conditions offered the opportunity for one of the pursuers to jump into the fray.
Four-time Major Champion Brooks koepka he made a late charge for his first British Open with a round of 65-under-five to reach eight-under, but was left to regret his two-over-par score on Saturday.
“I didn’t play well enough on Saturday,” Koepka said. “It doesn’t really matter what ends today. I didn’t get a chance to win and that’s disappointing.”
South African Dylan Frittelli, who was only called up as the 11th reserve on Monday, finished fifth with nine under par.
World number one Dustin Johnson finished tied for eighth at seven under par.