CHARLOTTE, North Carolina, May 05, 2013 (AFP) - Ernst, ranked outside the top 1,200 at the start of the week and in the field as the fourth alternate, birdied the 72nd hole of regulation to seize a share of the lead, then returned to the 18th for the first hole of the sudden-death playoff.
Lynn, who was also seeking a first US PGA Tour title, hit his drive into the rough along the bank of the stream left of the fairway. He found a greenside bunker with his second shot, while Ernst was on the fairway and landed his approach 15 feet from the pin.
Lynn's bunker shot flew the green and he chipped seven feet past.
Ernst's birdie putt lipped out, but his tap-in for par was enough for the victory that also brought him an invitation to next week's prestigious Players Championship.
The two finished four rounds at Quail Hollow on eight-under 280. In a rainy final round in which the tee times were brought forward to avoid the worst of expected inclement weather, both Ernst and Lynn had four birdies and two bogeys as both shot two-under 70.
"This feeling is unbelievable right now," said Ernst, a 22-year-old Californian playing just his ninth US PGA Tour event.
"Just at the beginning of the week not even knowing I was going to be in the field. I was fourth alternate last time I heard, and then a couple people dropped out so I got in."
Ernst's four-foot birdie putt at the last hole of regulation was just one of four birdies at the 18th all day.
"Winning was not on my mind," he said of his mindset at the 72nd hole. "On 18, I had 190 something, I don't know. I hit a six-iron and choked up a little bit and hit a draw in there.
"I was trying to hit it as close as I possibly could, yeah. I knew Dave (Lynn) was at eight (under), so I didn't know what Phil was or where he was at. So I was just trying to get tied with David at eight."
Three-time Masters champion Mickelson, who shared the overnight lead with Nick Watney, arrived at the 16th hole with sole possession of the lead, but he bogeyed 16 and 17 and didn't even make the playoff.
His approach at 18 was in the back fringe and he missed a 19-foot birdie attempt that would have given him a share of the lead.
Mickelson finished with a 73 for third place on 281.
England's Lee Westwood and Sweden's Robert Karlsson both posted last-round 72s to share fourth on 282.
World number two Rory McIlroy of Northern Ireland, who notched his breakthrough US PGA Tour win at Quail Hollow in 2010 and lost in a playoff last year, carded a second straight 73 to finish in a group sharing 10th place on four-under 284.
Leading final-round scores on Sunday in the US PGA Tour $6.7 million Wells Fargo Championship at Quail Hollow (x-won at first playoff hole, USA unless noted, par-72):
280 - x-Derek Ernst 67-71-72-70, David Lynn (ENG) 71-68-71-70
281 - Phil Mickelson 68-67-73-73
282 - Lee Westwood (ENG) 70-68-72-72, Robert Karlsson (SWE) 69-72-69-72
283 - Kyle Stanley 74-68-73-68, Bo Van Pelt 74-70-68-71, Kevin Streelman 68-72-71-72, Ryan Moore 67-75-68-73
284 - Ross Fisher (ENG) 70-71-73-70, Charles Howell 72-72-69-71, Vaughn Taylor 70-72-71-71, Rory McIlroy (NIR) 67-71-73-73, Brian Harman 70-70-71-73, Nick Watney 67-70-71-76
285 - Luke List 71-75-71-68, Henrik Norlander (SWE) 74-70-69-72, Lee Dong-Hwan (KOR) 72-71-69-73, Sergio Garcia (ESP) 72-68-72-73, D.A. Points 71-69-71-74, George McNeill 69-68-72-76
286 - John Rollins 69-74-72-71, Jimmy Walker 71-72-73-70, Scott Gardiner (AUS) 70-67-76-73, John Merrick 74-71-68-73, John Senden (AUS) 70-73-67-76
287 - Robert Garrigus 67-72-75-73, Brian Davis (ENG) 72-74-69-72, Shawn Stefani 69-73-72-73, Robert Allenby (AUS) 76-70-71-70, Lucas Glover 68-71-73-75