After three calm and sunny days, the wind picked up off the Moray Firth coastline and turned a beautiful course into a beast. Only five players shot lower than 70 on Sunday.
Mickelson was in more trouble when his chip from just off the third green came up short and rolled back down the hill to his feet. He was then five shots off the leader.
"In conditions like these, there are going to be mistakes," Mickelson said. "There isn't a perfect round in these conditions."
Urged on by a Scottish crowd that has really taken to the American — a long-time supporter of this tournament and a lover of Scotland and its courses — he birdied the next three holes and then picked up shots at Nos. 11, 12, 14 and 16 to take the sole lead, after briefly sharing it with Stenson and Hansen.
With the two Scandinavians dropping shots coming home, Grace became Mickelson's closest challenger and his illustrious rival handed him a reprieve to take the event beyond 72 holes.
"I was so mad at myself after mentally losing my focus," Mickelson said.
They were forced to wait to tee off again with organizers having removed the tee markers on the 18th, clearly confident Mickelson would finish the job the first time around.
Mickelson held his nerve in the playoff, though, with his approach shot bringing roars of approval from fans.
"This tournament, it's special to me ... to play well at the Home of Golf really means a lot to me," he said.
Hansen, who had missed the cut in 12 of his 17 previous tournaments in his rookie season, had threatened to produce one of the most remarkable comeback wins.
He found the bushes twice on the second fairway — forcing him to take two penalty drops — and then chipped short of the green with his sixth to wind up with a quadruple-bogey nine.
No one has ever won on the European Tour after making that score in his final round and he couldn't alter that stat, despite reeling off seven birdies on his next eight holes to briefly take the lead.
Like Stenson, he faded away in the home stretch.
DIVOTS: Martin Laird shot 68 for the lowest round of the day, finishing in a tie for fifth and as the highest-placed Scot. ... Mickelson's win allowed Steve Stallings of the United States to get into the British Open as an alternate.