A stroke off the pace heading into the final round, American Mahan sparked his sizzling run by knocking in a five-footer at the par-four 11th and effectively sealed the win at the par-five 17th, where he sank a slick 22-foot putt from above the hole.
He could afford the luxury of a bogey at the last, after pushing his tee shot into a tree line, chipping out and missing the green with his third, as he closed with a six-under-par 65 at Ridgewood Country Club to post a 14-under total of 270.
Stuart Appleby also signed off with a 65 in the opener of the PGA Tour’s four-event FedExCup playoffs, finishing joint second at 12 under with fellow Australian Jason Day (68) and American Cameron Tringale (66).
Veteran American Jim Furyk, co-leader after the third round with Day, mixed five birdies with four bogeys on the way to a 70 as he slipped back into solo eighth place at 10 under.
Mahan, who was congratulated on the edge of the 18th green by his wife Kandi and daughter Zoe Olivia, was thrilled to clinch his first playoff event and end a title drought dating back to the 2012 WGC-Accenture Match Play Championship.
“I haven’t played my best this year so to get a win at an event like this, and the timing that it was (in the playoffs), feels unbelievable,” a beaming Mahan told CBS Sports.
“I am extremely proud of myself right now. I’ve had some success the last couple of weeks, my game was starting to come around and I was making some putts, making some 20-footers, getting up and down here and there.
“I knew this was around the corner but to do it, and to do it today with a 65, feels great.”
Mahan, whose previous best finish this season was a tie for fourth at the Phoenix Open in February, was taken aback by the unexpected but very welcome arrival at the course by his wife and daughter.
“It was great,” smiled the 32-year-old American, who moved to the top of the FedExCup points standings with his victory on Sunday.
“There was a lot of going on in my head right there (on the 18th green), and I never thought they would be there but it’s always great to see them. They make every day a little bit better.”
Tringale, who last week disqualified himself from the PGA Championship for signing an incorrect scorecard, was in a buoyant mood after celebrating his 27th birthday on Sunday by shooting a final-round 66.
“It was a lot of fun out there,” the Californian said after recording his best finish on the PGA Tour. “Being my birthday, I was going to have a great day no matter what and that’s been my attitude all week.
“I really didn’t feel nervous out there, I just felt comfortable, like I was in control of what was going on.”
Overnight pacesetters Day and Furyk duelled for the tournament lead on the front nine before Mahan, Appleby and Tringale made their moves as prime contenders after the turn.
As both Day and Furyk faltered, Mahan was the first player to reach 13 under, sinking a 10-foot birdie putt at the par-three 15th, before birthday boy Tringale drew level by knocking in a six-footer to birdie the par-five 17th.
A superb approach by Mahan to three feet at the 16th set up his sixth birdie of the day and the tournament was all but over when he coaxed in his 22-footer on the 17th green.
Northern Irish world number one Rory McIlroy, who arrived here on a sizzling three-tournament winning streak that included major triumphs at the British Open and PGA Championship, ended his week with a 70 for a five-under total of 279.
Second-ranked Australian Adam Scott, who had been tied for the second-round lead at Ridgewood, finished at seven under after closing with a 68.
The top 100 players on the FedExCup points list after The Barclays qualify for next week’s Deutsche Bank Championship in Boston where the leading 70 will advance to the Sep. 4-7 BMW Championship.
(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Frank Pingue)