At just 24, Morikawa claimed his second major thanks to a bogey-free round of 66 to finish on 15 under par.
The American had never even played in links conditions until last week’s Scottish Open when he tied for 71st, the worst finish of his professional career.
But the world number four showed no sign of inexperience as he revelled in the sweltering conditions as temperatures hit 30 degrees celsius in Sandwich on England’s south-east coast.
Overnight leader Louis Oosthuizen‘s bogey at the fourth allowed Morikawa to move into a share of the lead and he never looked back after a two-shot swing at the par-five seventh.
A dream debut 👏Congratulations, Collin Morikawa, Champion Golfer of the Year 🏆 #TheOpen https://t.co/x1TfIPewYI
— The Open (@TheOpen) 1626629598000
Oosthuizen, who finished at 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 consecutive birdie putts to hit the turn at 14 under.
The 2020 US PGA champion needed his putter to stay hot on the back nine just to save par at 10, 11 and crucially at 15 with Spieth breathing down his neck.
But by that time, he had moved to 15 under with another long-range putt at the par-five 14th.
Three solid pars coming home made Morikawa the first British Open debutant to win since Ben Curtis’ shock success also at Royal St. George’s 18 years ago.
Spieth was left to regret his sloppy finish to Saturday’s third round when bogeys at the final two holes saw him start the day two behind Morikawa.
The 2017 British Open champion slipped back to seven under after dropping another two shots at the fourth and sixth before roaring back in a manner that suggests he will not be waiting long to add to his three majors.
An eagle at the seventh launched Spieth’s challenge and he completed the final 12 holes in six under par with further birdies at the ninth, 10th, 13th and 14th to put the pressure on Morikawa.
But Morikawa never flinched to become the first man to win two majors on debut at that event.
Rahm also showed why he was the pre-tournament favourite after landing his first major at last month’s US Open with a brilliant 66 that could have been far better but for a better day with the putter.
The Spaniard needed an eagle on seventh to get him going, but just ran out of holes after four consecutive birdies between 13 and 16.
Oosthuizen’s wait for a second major goes on as he finished in the top three for the eighth time since his sole success in the 2010 British Open at St Andrews.
The pristine conditions offered the chance for one of the chasing pack to launch themselves into contention.
Four-time major champion Brooks Koepka made a late charge for his first British Open with a five under par round of 65 to get to eight under, but was left to rue his two over par score on Saturday.
“I didn’t play good enough Saturday,” said Koepka. “Doesn’t really matter what I finished today. I didn’t have a chance to win and that’s disappointing.”
South African Dylan Frittelli, who was only called into the field as 11th reserve on Monday, finished fifth on nine under.
World number one Dustin Johnson finished in a tie for eighth on seven under.