At just 24, Morikawa won his second major with a bogey-less round of 66 to finish at 15 under par.
The American had never even played in tie conditions until last week’s Scottish Open where he tied for 71st, the worst finish of his professional career.
But the world number four showed no signs of inexperience as he reveled in the sweltering conditions as temperatures soared to 30 degrees Celsius in Sandwich on England’s south-east coast.
Night leader Louis Oosthuizen’s bogey in the fourth gave Morikawa a share of the lead and he never looked back after a two-stroke swing in the seventh by five.
A dream start Congratulations, Collin Morikawa, Champion Golfer of the Year 🏆 #TheOpen https://t.co/x1TfIPewYI
– The Open (@TheOpen) 1626629598000
Oosthuizen, who finished 11 under in a share of third with Jon Rahm, twice found bunkers around the green to drop another shot.
Meanwhile, Morikawa drained the first of three straight birdie putts to hit the 14-under turn.
The 2020 PGA United States Champion needed his putter to stay warm on the back nine just to save par at 10, 11 and especially 15 with Spieth breathing down his neck.
But by then he was down to 15 under with another long distance putt at the 14th par five.
Three strong home starts made Morikawa the first British Open debutant to win since Ben Curtis’ shocking success also at Royal St. George’s 18 years ago.
Spieth regretted his sloppy finish in the third round on Saturday when bogeys at the last two holes saw him start the second day behind Morikawa.
The 2017 British Open champion fell back to seven under after losing two more shots in the fourth and sixth before roaring back in a way that suggests he won’t wait long to add to his three majors.
An eagle in the seventh threw up the challenge for Spieth and he finished the last 12 holes in six under par with further birdies in the ninth, 10th, 13th and 14th to put pressure on Morikawa.
But Morikawa never flinched to become the first man to win two majors on his debut at that event.
Rahm also showed why he was the pre-tournament favorite after clinching his first major at last month’s US Open with a brilliant 66 that could have been a lot better but for a better day with the putter.
The Spaniard needed a seventh-place eagle to get him started, but just ran out of holes after four straight birdies between 13 and 16.
Oosthuizen’s wait for a second major tournament continues as he finishes in the top three for the eighth time since his only success at the 2010 British Open in St Andrews.
The pristine conditions offered the chance for one of the pursuers to engage in the fight.
Quadruple major champion Brooks Koepka carried out a late charge for his first British Open with a round of 65 under par five to reach eight under par, but must have regretted his score of two over par on Saturday.
“I didn’t play well enough on Saturday,” Koepka said. “It doesn’t matter what I finished today. I didn’t have a chance to win and it’s disappointing.”
South African Dylan Frittelli, who was only called to the field as an 11th reserve on Monday, was fifth with nine under.
World number one Dustin Johnson finished tied for eighth in seven under.