Johnson looked set for victory as she surged through the field with a course record 65 - but her only bogey of the day at the 18th hole proved costly. At the par five, she went for the green with her second shot. But she failed to make the carry over the water and a bogey six allowed Gronberg Whitmore to draw level.
The 34-year-old Swede - who had her English husband John acting as her caddie - was two behind with two to play. She birdied the 17th from 10 feet and made a safe par at the last in a 69, and a birdie at the second extra hole - the 18th - secured the first prize and her first Tour title since her maiden win, in the 1989 Rome Classic.
Four behind at the start, Johnson surged to the turn in 29 - equalling the tour record - and had six birdies and an eagle at the third in the spectacular round.
"In retrospect, I should have laid up at the 18th," she said afterwards. "I am gutted, but I had been striking the ball so well and it was only bad shot. It was the wrong time to hit it."
In her own event, Laura Davies closed with a 74 which left her in a tie for 13th place.Reuse content