Montgomerie made a charge of his own. He birdied Nos. 8, 9 and 10 and, with precise iron shots and clutch putting, also made birdies at 12, 14 and 15 to pull away. He said the birdies in the middle of the round were the key.
"I was caught," he said. "It was between seven or eight players that could actually win there. Those birdies were the key."
Montgomerie will head home to Scotland for a few weeks with a first-place check of $378,000 and his name will go on the Alfred S. Bourne Trophy. The win also netted him a lifetime exemption to the Senior PGA Championship, and 2014 exemptions for the PGA Championship, Senior British Open and U.S. Senior Open.
Watson, who made a bid to be the oldest player to win a senior event of any kind, had five consecutive pars to end his round while missing several birdie chances. He hit 17 of 18 greens in regulation.
"It was one of the best rounds from tee to green that I played in years," Watson said. "It was really, really good, but the putter felt like a snake in my hands. I missed a lot of short putts today. It could have been a much better scoring round of golf."
Bernhard Langer, playing with Montgomerie for a fourth consecutive round, hit his tee shot at No. 11 in the hazard right of the green and ended up making double bogey on the 140-yard par 3. It put him four shots behind Montgomerie at the time.
Montgomerie is the third Scot to win senior golf's oldest championship, and the first since Jock Hutchison won his second Senior PGA in 1940.