LOS ANGELES, 23 Feb 2015 (AFP) - After draining the putt at the third playoff hole, the par-three 14th, South Korean-born American Hahn could barely watch as Johnson missed his effort from 12 feet.
England's Casey had been eliminated on the previous hole, Riviera Country Club's testing par-four 10th.
In the best shape off the tee, Casey's second shot from a groomed area below the green left him 13 feet and he missed the putt.
Johnson and Hahn, both in deep greenside rough, chipped closer, Hahn draining his nine-foot birdie putt before Johnson made his own four-footer.
"Amazing. I birdied two playoff holes, against Paul Casey and Dustin Johnson two, great competitors," Hahn said. "I'm lucky to be here now."
Hahn's breakthrough win comes three weeks before another life-changing event, with his wife due to give birth to their daughter.
"My golf life changes, but more importantly, I'm going to have another addition to the family, and I'm just looking forward to it," said Hahn, whose previous best finish on the PGA Tour was a tie for third at Pebble Beach in 2013.
Hahn, perhaps best known previously for his Gangnam Style dance at the Phoenix Open, has now booked a Masters berth and a two-year exemption on the PGA Tour.
"Obviously winning this tournament is the best thing that's ever happened to me. But in about three weeks, I'm sure there's going to be another reason why I'm going to be even more happy and more grateful," he said.
Casey was the first of the leaders in the clubhouse on six-under, one shot off the lead on the course after closing bogey-birdie-birdie-bogey.
Johnson missed a nine-foot birdie putt at the final hole of regulation.
"I played great today, I really did," Casey said. "I'm not disappointed in any shape or form."
He could only take his hat off to Johnson and Hahn for their shots from the rain-soaked rough at the second playoff hole.
"Those guys played phenomenal recoveries, absolutely brilliant, both of them," Casey said. "Dustin's was the toughest of the bunch, shows he doesn't have just great length off the tee. He has great hands as well."
On a day when seven players held at least a share of the lead at some point, Casey, Johnson and Hahn finished 72 holes tied on six-under 278.
Casey, a 13-time winner on the European Tour who was seeking just his second US PGA win, carded a 68 while Johnson and Hahn both signed for 69s.
Spain's Sergio Garcia was clinging to a one-stroke lead with two to play, but closed with back-to-back bogeys to miss the playoff by a stroke.
Garcia finished tied for fourth with Japan's Hideki Matsuyama, rising US star Jordan Spieth, and major winner Keegan Bradley on five-under 279.
- Goosen, Singh falter -
Overnight leader Retief Goosen, the two-time US Open champion seeking his first US tour win since 2009, led by as many as two, but found two double bogeys -- at the par-four eighth and the par-four 13th -- too much to overcome.
Even two closing birdies left him on four-over 75 and tied for eighth on 280 with South Korean Bae Sang-Moon, Canadian Graham DeLaet and American Kyle Reifers.
Fiji's Vijay Singh, a three-time major champion seeking to celebrate his 52nd birthday Sunday with his first win since 2008, saw his hopes evaporate for good with a bogey, double-bogey and bogey at 15, 16 and 17. He finished with a 72 for 281.
For Johnson, it was another strong showing in his third tournament back since a self-imposed leave of absence to deal with personal issues.
He was tied for fourth at Pebble Beach last week.
"It's good to be back and good to be in the hunt and having a chance to win," Johnson said.
Leading final-round scores on Sunday from the $6.7 million US PGA Tour Northern Trust Open (x-won at third playoff hole, USA unless noted, par-71):
278 - x-James Hahn 66-74-69-69, Dustin Johnson 70-72-67-69, Paul Casey (ENG) 70-69-71-68
279 - Hideki Matsuyama (JPN) 70-72-70-67, Keegan Bradley 73-68-70-68, Jordan Spieth 69-70-70-70, Sergio Garcia (ESP) 71-69-68-71
280 - Kyle Reifers 72-70-71-67, Retief Goosen (RSA) 66-70-69-75, Graham DeLaet (CAN) 70-67-70-73, Bae Sang-Moon (KOR) 71-71-66-72
281 - Blayne Barber 74-71-71-65, Vijay Singh (FIJ) 66-74-69-72
282 - William McGirt 68-74-73-67, Matt Jones (AUS) 70-72-72-68, Brendan Steele 74-70-69-69, Brendon Todd 69-72-71-70, Bubba Watson 70-69-70-73, Jim Furyk 71-70-68-73
283 - Hunter Mahan 75-67-71-70-74, Carlos Ortiz (MEX) 67-73-68-75
284 - Nick Watney 66-74-74-70, Scott Langley 73-71-71-69, Adam Hadwin (CAN) 73-71-71-69, Noh Seung-Yul (KOR) 71-69-73-71, Morgan Hoffmann 69-75-66-74, Ryan Moore 69-68-72-75, Vaughn Taylor 74-68-68, J.B. Holmes 70-69-69-76