It is the 50-year-old's sixth major victory, moving him into a tie for 12th on the all-time leaderboard. 

Mickelson began the final round ahead by one stroke, and a two-shot swing on the first hole saw fellow American Brooks Koepka move into the lead.

Yet Koepka made a double-bogey seven on the next hole, a par five, and Mickelson carded a birdie to reclaim the lead.

From there he never lost it, with chipping in from the sand on the fifth hole a highlight from an up-and-down front nine.

Koepka fell away badly, with three bogeys in four holes to start the back nine effectively ending his hopes, and South Africa's Louis Oosthuizen was not able to apply much pressure either, ultimately carding a one-over-par 73.

That meant that despite back-to-back bogeys on 13 - where he found the water - and 14, Mickelson still had room to play with.

Two shots ahead on the final hole, the left-hander produced an excellent approach shot from the rough to put the result beyond doubt, and two-putted for par once an enthusiastic crowd had been cleared.

Mickelson carded a one-over 73, was six strokes under par for the tournament in South Carolina - and two ahead of both Koepka and Oosthuizen.

Four players were tied for fourth on two under, including Ireland's Padraig Harrington - himself 49 and a former winner of the Professional Golfers' Association of America's (PGA) flagship event.

"This is just an incredible feeling," Mickelson said after winning his first major since 2013.

"I just believed that it was possible yet everything was saying it wasn't and I hope that others find that inspiration.

"It may take a little extra work and harder effort to maintain the physicality, or maintain the skills, but gosh is it worth it in the end.

"I just love this game and I love what I do and I love the challenge of competing against such great players."