The American, who had former European Solheim Cup star Sophie Gustaffson on her bag at the Buckinghamshire Golf Club??this week, closed with a five-under 67 which gave her the clubhouse lead on 12 under par.
A play-off looked imminent when Maguire, the world amateur No 1, rolled in her sixth birdie putt of the day at the penultimate hole to claim a share of the lead with Allen.
But Maguire’s tee shot at the par-three 18th bounded over the green and into a bunker, and she splashed out to eight feet before her putt for par grazed the edge of the cup on the low side.
With overnight-leader Caroline Masson out of the running after an erratic day, Allen was confirmed as champion when Nontaya Srisawang failed to make the hole-in-one she needed to force extra holes.
Ace for Dame Laura
English legend Dame Laura Davies had earlier lit up the final round with a superb hole-in-one at the 171-yard seventh, using an eight-iron to record the 11th ace of her glittering professional career.
The 51-year-old went on to card a commendable 67 to earn a share of seventh place on eight under with fellow veteran Trish Johnson, who returned a flawless six-birdie 66 to post the low round of the final day.
But the tournament belonged to Allen, who surged into contention with a superb front-nine 31 which featured four birdies in five holes, and she picked up another shot at the 14th before making her only mistake of the day at the next.
Allen held her nerve to negotiate the final three holes in level par and remain at 12 under, while Maguire had to be content with outright second after mixing six birdies with three bogeys in her third 69 of the week.
Srisawang’s level-par 72 was good enough to give her third on 10 under, but it was a poor day for Masson as her excellent form of the first three rounds deserted her.
The German led by two overnight, but she bogeyed two of the first five holes which set the tone for an error-strewn three-over 75 which left her three strokes off the pace.
Meanwhile, Charley Hull’s miserable weekend continued as she followed Saturday’s 77 with a 78 that included a double-bogey at the 13th and a triple-bogey at 15 – leaving her outside the top 40 on two over par.