WASHINGTON, 5 June 2016, (AFP) - The 36-year-old McGirt rolled in a six-foot par putt on the second playoff hole to cap off a bogey-free day and edge fellow American Jon Curran after they both finished the fourth round at 15-under 273.
"It was an unbelievable struggle all day," he said. "My putter saved me this week.
"I fought my golf swing but I just tried to hold onto it."
McGirt posted his first victory since turning pro 12 years ago. He booked a spot in the US Open at Oakmont in two weeks, along with a three-year PGA Tour exemption.
He has appeared in just one major previously in his career, missing the cut in the 2012 PGA Championship.
McGirt hit his winning putt then punctuated the victory with a right fist pump before the ball even dropped into the cup on the 18th hole at the Muirfield Village Golf Club.
McGirt hopes that this is the breakthrough he was looking for to become a consistent contender on the Tour.
"I have been in this position before," he said. "I feel like I learned something from it. In the past my tendency was to get fast -- my walking, my swing. I tried to slow down a bit today."
McGirt shot a one-under 71 Sunday, while Curran shot a 70.
McGirt became the sixth player to have his first career win come at the Memorial, including the last two -- Hideki Matsuyama of Japan and David Lingmerth of Sweden.
McGirt and Curran had a combined 218 PGA Tour starts without a victory.
Curran said he was able to hang in there despite not having his best array of shots.
"To be honest, I haven't felt great off the tee all week but I was able to battle through that," Curran said. "It doesn't matter how you feel. If you can just perform and trust yourself and just keep plugging away."
It was the third straight year that the tournament hosted by Jack Nicklaus went to extra holes.
Dustin Johnson finished one shot out of the playoff at 14-under 274. He shot a final-round 71 that included five birdies.
Northern Ireland's world number three Rory McIlroy carded a 68 to head a group sharing fourth on 13-under 275. He was joined by Americans JB Holmes (69), Gary Woodland (73) and Matt Kuchar (73).
The field was one of the strongest of the season, featuring the top five players in the world.
World number one Jason Day of Australia carded a final-round 74 that left him tied for 27th on 279.
World number two Jordan Spieth closed with a 73 to finish in a tie for 57th on 285.
Leading scores after Sunday's final round of the PGA Tour's Memorial Tournament at Dublin, Ohio (x-won at 2nd playoff hole. Par 72, USA unless noted):
273 - x-William McGirt 70-68-64-71, Jon Curran 68-67-68-70
274 - Dustin Johnson 64-71-68-71
275 - Rory McIlroy (NIR) 71-66-70-68, J.B. Holmes 71-68-67-69, Gary Woodland 68-65-69-73, Matt Kuchar 66-66-70-73
276 - Patrick Reed 68-71-69-68, Keegan Bradley 68-69- 70-69, Kevin Streelman 67-68-69-72,
277 - Marc Leishman (AUS) 69-71-69-68, Tony Finau 70-69-70-68, Charl Schwartzel (RSA) 68-69-72-68, An Byeong-Hun (KOR) 71-70-69-67, Roberto Castro 70-70-71-66, John Senden (AUS) 69-70-68-70, Webb Simpson 69-70-66-72, Emiliano Grillo (ARG) 67-66-70-74, Adam Hadwin (CAN) 70-66-67-74
278 - Smylie Kaufman 71-67-71-69, Kyle Reifers 71-67-71-69, Brendan Steele 65-67-75-71, Matt Jones (AUS) 71-68-68-71, Phil Mickelson 68-69-69-72, Robert Streb 68-73-64-73, Soren Kjeldsen (DEN) 71-69-64-74
279 - David Lingmerth (SWE) 68-73-68-70, David Hearn (CAN) 66-73-72-68, John Huh 69-69-67-74, Jason Day (AUS) 66-71-68-74, Scott Brown 69-67-69-74, Zac Blair 69-67-68-75