Both players scored final-round 69s, three under par, to finish level on 17-under at Castle Stuart, Inverness. Mickelson three-putted the last regulation hole for a bogey 6 as he missed a victory chance.
“This is really cool, this golf course is just wonderful, and to come out on top really means a lot,” Mickelson told Sky Sports. “I was so mad at myself for losing my focus,” he said of the three-putt. “I came out pretty focused” for the playoff, he added.
The American clinched his first individual win in Britain at the opening extra hole with a birdie after pitching to within inches.
Four-time major winner Mickelson, 43, recovered from a double-bogey six at the first hole today and went on to make seven birdies in the round.
Denmark’s JB Hansen had a quadruple-bogey nine at the second hole but rallied to finish joint third with Sweden’s Henrik Stenson on 15-under.
Mickelson picked up a check for 579,080 euros ($758,000).