Maggert, who tasted victory at the Regions Tradition in Alabama last month, made six birdies and a bogey during his final round of 65 to finish on 10-under, having begun the day as the 54-hole co-leader with Bernhard Langer.
The 51-year-old's victory is his third on the Champions Tour, the same number of PGA Tour titles he won over a span of more than two decades, and he takes home 675,000 dollars and an exemption into next year's U.S. Open at Oakmont.
Defending champion Colin Montgomerie was unable to become the first back-to-back winner since Allen Doyle in 2005 and 2006 as he recorded four birdies during his round of four-under par 66 to finish second.
Langer and New Zealand's Grant Waite were tied third on the leaderboard after carding final rounds of 67 and 68 respectively.
Tom Watson was in contention to become the oldest winner of a senior major at the age of 65 when he started the final round just one stroke off the lead. However, the American finished tied seventh after an up and down round of 69, which consisted of four birdies and three bogeys.
Billy Andrade, who finished tied fifth, registered the lowest score of the round with a 63, just one stroke shy of equalling Loren Roberts' record for the lowest 18-hole score in any USGA Open championship.