Westwood entered the BMW PGA Championship at No. 1, but it was Donald's approach shot on No. 18 -- the first playoff hole -- that will send him to the top when the new golf rankings are released Monday.
While Donald's approach landed a few feet from the hole Sunday, Westwood sent his shot into a water hazard in the tournament at Wentworth. And just like that, Donald was a winner and No. 1 in the world.
"Sounds pretty good, doesn't it?" asked Donald, who joins Westwood and Nick Faldo as the only Englishmen to hold the No. 1 spot since rankings were introduced in 1986. "It's an amazing accomplishment. It's something I will be very proud of when I look back. I will savor this moment, it's very special. I think I will stay the same person, I hope I do."
The win was only the second in stroke play for Donald in the past five years. He won the Madrid Masters on the European Tour last year. In February, he beat a top field in winning the Match Play Championships in Marana, Ariz.
"I'm looking forward to the challenge of staying No. 1," Donald said. "I know Lee and Martin (Kaymer) will be chasing me very hard."
Having trailed co-leaders Matteo Manassero and Donald by two strokes entering the final round, Westwood shot a 3-under 68. Donald shot 70 and both finished at 6-under 278.
But his consistency in finishing in the top 10 for the past nine tournaments ensured his first playoff victory on either the European or PGA Tour will give him the No. 1 ranking.
Donald said Westwood's water-bound approach in the playoff was more bad luck than poor execution.
"It wasn't a bad shot, just had a little bit too much spin on it. It was just unfortunate, you don't like to see that," Donald said. "Lee is a champion and has been a great No. 1 for European golf."
After rounds of 72 and 72, Donald began the final day by going bogey-bogey, visibly blanching after duffing his chip at the par-3 No. 2 from just off the green.
Manassaero double-bogeyed No. 3 after needing two shots to reach the green from an adjacent bunker. Donald birdied the next hole to draw even.
Westwood, however, made four birdies in nine holes after a bogey on No. 3 to sit on top of the leaderboard.
Another birdie on the difficult No. 15 green was made possible by one of the shots of the tournament. Having missed the fairway with his driver and receiving a free drop, Westwood carved a 7-iron with draw that rolled within 4 feet of the hole, the putt moving him two strokes ahead of Donald.
That lead evaporated at No. 16.
Westwood three-putted after having dropped his club in his follow through from the tee. Donald, in the next group, landed his approach within a foot of the hole -- and the top two players in the world were even again.
Down the stretch, Donald and Westwood mixed the substandard with the sublime.
Donald's birdie putt lipped out on No. 17 after a brilliant rescue shot from an approach that hit a tree. He landed his tee shot into the bunker at No. 18, not long after Westwood had sent his 5-wood into the gallery.
Both players made par and headed back to No. 18 for the playoff that would result in a new No. 1.