The American wrestled hold of the lead with three birdies to finish the front nine and held off the charging Jordan Spieth to win by two strokes on the Kent coast. The 24-year-old, who won his first major on his US PGA Championship debut last year, becomes the first Open debutant to win since Ben Curtis did so at this course in 2003.
"This is one of the best moments of my life," he said. "Look at all these fans. To see some of the best crowds I have ever seen, I'm looking forward to making my trip every year."
Morikawa's four-under 66 saw him win on 15 under, with 2017 champion Spieth also recording a final-day 66 to finish as his fellow American's nearest challenger.
US Open champion Jon Rahm birdied four of his last six holes to card a 66 and finish joint third on 11 under with overnight leader Louis Oosthuizen, who closed with a 71. Scotland's Robert MacIntyre finished as the top British player in a tie for eighth after a final-day 67 saw him finish at seven under par, two ahead of England's Paul Casey at five under.
Up to 32,000 spectators flooded through the gates on Sunday and were treated to a blockbuster final round as the only players to make the cut in all four of this year's men's majors finished as the top four in Sandwich. It was a fitting finale at Royal St George's as the venue belatedly got to hold the 149th edition of golf's oldest major after last year's championship was cancelled because of the Covid pandemic.