Montgomerie, who never won on the PGA Tour despite a decorated career in Europe, fended off a mid-round charge from Tom Watson to clinch the second major of the season on the over-50s Champions Tour.
The Scot carded 65 to finish at 13-under-par 271 and evergreen American Watson made four birdies in five holes from the turn to shoot 65 and claim second place.
"It's been a long time," Montgomerie told NBC TV. "I haven't won a 72-hole event in America until today. It's a very special moment for myself and my caddie. We've been trying, my god we've been trying.
"We've come close on a number of occasions. It's great to come here as a senior and finally break that duck."
Montgomerie, who turned 50 last year, has won 31 times on the European Tour, where he topped the order of merit a record eight times, including seven seasons in a row.
But his only previous victory in the United States was at the 1998 World Championship Match Play event, which at the time was unofficial.
Montgomerie teed off with a one-stroke lead on Sunday and effectively clinched the title with six birdies in eight holes, starting at the eighth.
Any lingering doubts about the outcome disappeared when his second shot into the par-four 18th sailed long and left of the green, only to strike the grandstand and ricochet back onto the putting surface.
Watson, 64, was the only player to make a serious run at Montgomerie, who paid tribute to the man who will captain the American Ryder Cup team against Europe in September.
"If his Ryder Cup team has his ambition we're in big trouble this year," Montgomerie quipped, before continuing to talk about how much the victory meant.
"It's a very difficult task to come from abroad and win in America. If you beat the Americans in America at any sport you've done really well.
"I'm very proud of myself, coming from little old Scotland and (winning) on American soil."
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