Mahan holed five birdies on the back nine to finish two strokes clear of Stuart Appleby, Cameron Tringale and Jason Day at the FedEx Cup play-offs opener in New Jersey.
The 32-year-old American, who withdrew from July's RBC Canadian Open as the second-round leader due to the birth of his daughter, is now top of the FedEx Cup standings.
He is also in the mix for an unlikely call-up for the Ryder Cup, which starts on September 28 in Scotland.
And the six-time PGA Tour winner concedes the possibility of featuring at his third Ryder Cup as one of Tom Watson's captain's picks, having been selected in 2008 and 2010, is playing on his mind.
"I have been thinking about it [Ryder Cup] for a few weeks," said Mahan.
"Just the unfortunate things that have happened to the guys on the team [injuries], seems like it's let a lot of guys in.
"I don't know who he [Tom Watson] is thinking about or what his process is but I knew a strong couple of weeks, a strong major and strong couple of playoff events could have a chance.
"Obviously a win helps a lot and playing well at the PGA helps a lot.
"I have no idea what he is thinking or if he has any sort of strategy but I think a win is a good step in the right direction."