Mahan bested a tight-knit field courtesy of five back-nine birdies at Ridgewood Country Club in New Jersey, to finish on 14 under overall in the FedEx Cup play-offs opener.
The 32-year-old won his sixth PGA Tour title, finishing with a six-under-par 65 despite a bogey at the 18th.
Australians Jason Day and Stuart Appleby joined American Cameron Tringale in a tie for second at 12 under, with the trio all inside the top 20 on the points list as a result.
Appleby carded a 65 to climb from the brink of elimination - the top 100 advance to next week's Deutsche Bank Championship - to 19th in the standings.
Tringale completed his solid week with a 66, reeling off three straight birdies from 15 through 17 before a final-hole bogey.
Day carded a final-round 68 to be seventh in the points list.
Tied for fifth were Americans Matt Kuchar and William McGirt, along with South Africa's Ernie Els on 11 under.
Els and McGirt carded respective week-best 66s, while Kuchar chipped in at 18 to finish with a 67.
Kuchar sits fourth in the FedEx Cup, while Jim Furyk is sixth after a final-round 70 that left him outright eighth.
Els climbed 52 spots in the points standings to 39th, as he gained some valuable FedEx Cup points.
Rounding out the top 10 were American quartet Patrick Reed, Rickie Fowler, Kevin Na and Morgan Hoffman on nine under.
Reed's undulating week continued - his scorecard read 71-66-73-65 - on Sunday with a six-under round, while Fowler finished strongly - making it back-to-back 67s for his fifth straight top-10 finish.
The unlucky story of the tournament was Troy Merritt, his tied for 46th finish took him to 101st in the points list - ruling him out of the season-ending play-offs.
Meanwhile, the fortunate player of the week did not even make the cut, Australia's Geoff Ogilvy bound for TPC Boston after ending the week in 100th spot.
Northern Ireland's Rory McIlroy is second, after consecutive 70s on the weekend saw him to a tied for 22nd finish.
It was the first time McIlroy had not won in four tournaments.