Pate, who had a one-shot lead over Quigley, who was in the clubhouse, also drove into the fairway at the closing hole. He heeded his caddie's advice and laid up short of the pond, a questionable decision since he only had 190 yards to the flag.
The 1976 U.S. Open champion wedged his third almost 30 feet past the hole and lagged his birdie try 3 feet short of the hole. Reid drained his eagle putt to get in the house at minus-8, but Pate still had a putt to win.
Pate's putt was never on line, right of the hole.
The trio headed back to 18 for the playoff and Pate hooked his tee shot into the left rough. Quigley and Reid both hammered their drives in the center of the fairway.
Pate had no choice but to lay up again. Reid hit his second to 25 feet, but Quigley's approach found the water as the ball came up a few feet short of the green. Pate hit his third to 7 feet and the championship now had come down to Reid and Pate.
Reid's bid for back-to-back eagles came up a little short as the ball moved right at the last second. He tapped in for birdie and since Quigley's fourth landed 6 feet from the hole, the Champions Tour iron man with 273 consecutive starts for which he was eligible, was officially eliminated.
Pate needed to convert his birdie putt to force a second hole. Reid could not look as Pate hit his putt, but when he opened his eyes, he saw Pate walking after his ball before it even reached the hole. Pate's birdie try didn't touch the cup, so it was Reid who hoisted the trophy.
'I'm shocked as anybody,' said Reid, who pocketed $360,000 for the major championship. 'I felt like I had a new life in the playoff. It's a funny thing.'
Pate was left afterward to think about whether he made a mistake on the 72nd hole.
'He felt like that was the shot, so I listened to him,' said Pate, referring to his caddie's suggestion. 'I'm the guy that has the last say-so. I probably should have hit a 5-iron. A bad mistake on my part, but they'll deal them again.'
Reid and Pate both shot final-round, 2-under 70s, while Quigley managed an even-par 72 on Sunday. The trio finished regulation knotted at 8-under-par 280.
Morris Hatalsky posted a 2-under 70 and took fourth at 4-under-par 284. Tom Jenkins carded a 1-under 71 and was the only other player under par at Laurel Valley. He came in at 1-under-par 287.
Several groups had to finish their third rounds Sunday morning as rain forced the suspension of play on Saturday. Sunday's final round featured an early fog delay, but other than that, it was smooth sailing.
Quigley and Pate dueled most of Sunday's final round and Quigley built a two-shot lead after his 16th hole. Pate answered with a 6-foot birdie putt at the 15th.
Quigley landed in a greenside bunker at 17 and blasted out to 4 feet. He missed the putt and now found himself tied with Pate at minus-8. Pate hit a 5-iron to a foot to set up birdie at the 17th and that gave him a one-shot lead.
Pate's miscue on the last opened the door for Reid, who played steadily on Sunday.
Reid began the final round two back of Quigley and struggled on the front nine with bogeys at five and eight. He birdied the 11th and 14th holes before the eagle at 18.
He polished off a 32 on the back nine and that conjured up memories of a past major. Reid owned a three-shot lead with three holes to play at the 1989 PGA Championship, but he found water at the 16th and bogeyed No. 17 to lose to the late Payne Stewart. Stewart shot a 32 on the back 16 years ago.
'He won the tournament with a great 32,' said Reid. 'I thought 32 might look pretty good on my card today. I said it probably won't win. I was hoping Jerry would make that putt. He played well enough to win and I felt bad for him.'
Allen Doyle (72), Mark McNulty (72), Des Smyth (68) and reigning U.S. Senior Open champion Peter Jacobsen (75) tied for sixth place at even-par 288.
Dave Barr (77), R.W. Eaks (77), Mark James (70) and Tom Kite (71) finished knotted in 10th at plus-1.