Gil Morgan, the overnight leader, and Bobby Wadkins, the first-round leader, had makable birdie chances at the 54th hole but missed their putts to fall one short of a possible playoff. Morgan managed a 2-under 70 on Sunday, while Wadkins, who battled back pain throughout the tournament, carded a 4-under 68.
Several players were jockeying for the lead down the stretch but Barr, who was as many as four shots off the lead as he made his way to the 15th hole, came out of the pack. He collected a birdie there and holed a seven-foot birdie putt at 16 to get to 7-under par.
Wadkins reached 9-under par after back-to-back birdies at 14 and 15. Wadkins, who played with Barr in the group ahead of Morgan, hit a 5-iron into the bunker at 17. He blasted out to 10 feet but missed the putt to drop back to 8-under par.
Barr also used a 5-iron at 17 but ran home a 35-footer for birdie to reach 8-under and tie Wadkins and Morgan in first place.
Morgan recorded only two birdies on Sunday through his first 16 holes. At the 17th, he drained a 15-foot birdie putt to go to 9-under par while his two closest competitors hit their drives on 18.
Wadkins drove into the first cut on the left side before knocking his approach through the green into a back bunker. Barr found the fairway off the tee but pulled his second shot left, near the grandstands.
Barr received a free drop and pitched his third 35 feet past the hole. Wadkins ran his bunker shot six feet past the hole, giving him a realistic chance at birdie and a tie for first.
Barr putted first and holed his second 35-foot birdie putt in as many holes. He was in at 9-under after four straight birdies but still had to watch as Wadkins putted and Morgan finished the hole.
Wadkins' birdie putt slid past the hole and he tapped in for par. He finished one shot behind Barr at 8-under par and was off to the fitness trailer to try and heal his aching back.
Morgan, now tied with Barr at 9-under, drove well right of the fairway and into a hazard. He took a drop back in the fairway and roped his third shot just short of the putting surface but in good shape to get up and down and force a playoff.
Morgan chipped his fourth shot just left of the hole but seven feet past. His par putt to force a playoff lipped out of the hole, giving Barr a dramatic come-from-behind victory.
'Standing there on 14, I didn't think it was feasible,' said Barr, who pocketed $217,500 for the win. 'Bobby was going along pretty good and I also got a bit of help from Gil. I don't think I've ever birdied the last four to win before.'
'I wish I could have finished a shot or two lower but I didn't,' said Morgan, a 21-time winner on the Champions Tour. 'I made a mistake at the last hole. That was the killer. I thought if I made par, I could salvage the situation. It was hard for me.'
Despite the back problems, Wadkins still had one goal in mind Sunday - win the tournament.
'The spasms were just as bad today,' said Wadkins. 'I was thinking about nothing but winning the golf tournament. I'm glad it's over but I have no regrets.'
Barr, a two-time winner on the PGA Tour, took a long road to the Champions Tour. He birdied the 72nd hole at Q-School to earn his card and is now the first player from north of the border to win on the 50-and-over circuit.
'So few of us in Canada can play at this level,' said Barr. 'It's satisfying to get the job done. It's been a long time. Finishing fifth at Q-School last fall really took some pressure off.'
Rodger Davis finished alone in fourth at 7-under par, followed by Hubert Green, who came in fifth place at 5-under-par 211.
Isao Aoki (69), Mike McCullough (69), Allen Doyle (71) and Tom Purtzer shared sixth place at minus-4.