Three strokes back to start the day, Mallon played brilliant, high-stakes golf Sunday to win the du Maurier Classic for her first LPGA major title since 1991.
Mallon's 3-under-par 69 gave her a total of 6-under 282, one shot better than Rosie Jones, who finished with a bogey-free 68.
Annika Sorenstam was third, at 284, after a 74.
Lorie Kane, the Canadian who was tied with Sorenstam for the lead after three rounds, wound up tied for fifth. She picked up her first career victory last week and was her nation's best hope for a homegrown champion in the last installment of this tournament.
Kane, Followed by huge galleries all day, ballooned to a 76 to finish the tournament at 2 under.
Du Maurier is a brand name for Imperial Tobacco and sharp restrictions on cigarette advertising taking effect in Canada next year will force the company to cut ties with an event it has sponsored since 1984.
The tournament, rotated among top Canadian courses, has been an LPGA major since 1979, but is likely to lose that status even if it can be saved by another local sponsor.
Mallon was masterful in a four-birdie, one-bogey round at the 6,403-yard Royal Ottawa Golf Club on a hot day.
Her most anxious moment came on the 427-yard, par-4 15th, when she rolled in a 7-foot par putt to stay at 6 under and keep a one-stroke lead over Jones.
Mallon followed that with a 15-foot birdie putt on the 16th for a two-shot lead. With that cushion, Mallon made a cautious bogey at the 18th to win by one.
It was her 13th career tour victory and second this year.
Mallon's only other major championship victories both came in 1991, at the LPGA Championship and the U.S. Open. She has been a fixture on leaderboards at majors for a decade and the du Maurier was the 17th time in her career she has finished in the top 10 in a major championship.
She finished second to Karrie Webb at this year's U.S. Open after a final-round 74.
Sorenstam and Kane figured to duel for the title, but that never happened. They entered the round at 6 under three better than Mallon and Diana D'Alessio but Kane bogeyed the first and third holes, Sorenstam the second and fourth.
Kane briefly took a one-shot lead with a birdie on the fourth hole, but then bogeyed the seventh and the eighth holes, while Sorenstam lst yet another stroke at the seventh.
Their staggering start both shot 3-over 39 on the front nine let others seize the momentum.
Kane kept her fans' hopes alive, briefly, with a birdie on the 10th from about 10 feet, but then she bogeyed the par-4 13th by failing to get up and down from the rough left of the fringe.
Then she hooked her drive on the par-5 14th out of bounds, made a bogey and fell to 2 under.
Kane, of Prince Edward Island, was trying to become the second Canadian to ever win an LPGA major. Sandra Post won the 1968 LPGA Championship.
Jones started the day at 1 under, but birdied Nos. 3, 6, 8 and 10 to get into second place behind Mallon. She made a sensational par save on the par-4 16th after missing the green left with her approach and chipping to within about 20 feet of the hole.
That kept her one stroke behind Mallon, but Jones missed birdie putts of 15 feet or longer on Nos. 17 and 18 that could have put more pressure on Mallon.
Jones has never won a major championship.
Tour rookie D'Alessio was in contention all week despite some wildly erratic driving and iron play. She got within a shot of Mallon at 5 under, before double-bogeying the par-3 11th hole to fall back. She shot a 72 Sunday to finish fourth at 3 under.
Juli Inkster, seeking her second major title of the year, never had consecutive birdies during her final-round 72. She finished tied with Kane.
Divots: Mallon won $180,000.
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