The Calgary native snapped a 14-tournament winning streak for players from the United States. Heffernan's win marked the first time since Matt McQuillan's victory in the summer of 2005 at the Edmonton Open a non-American won on the circuit.
'This feels just awesome,' said Heffernan. 'The first one is always special, especially the way I did it.'
Heffernan, the overnight leader, only managed an even-par 72 on Sunday, including a birdie at the 18th hole to force the extra session. He tied Mackenzie, who shot a final-round, 4-under 68 at 18-under-par 270 at Quatre Domaines Golf Club.
Heffernan bogeyed the 17th to fall one back, but had the par-5 final hole to make birdie and take it to extra holes. He had 285 yards to the flag and hit a 3-wood to the back of the green. Heffernan chipped to 4 feet and converted to set up a date with Mackenzie on the same hole.
Heffernan tried to get home in two again at the 18th, but landed in a greenside bunker. He blasted out to four feet, and when Mackenzie missed his 15-foot birdie putt, Heffernan drained his putt for the win.
Is the next step to the PGA TOUR?
'Maybe this is what I need to get me there,' admitted Heffernan. 'I knew I could get it done. It's just taken me a little longer.'
Mackenzie will have to wait for his first Canadian Tour win.
'I've been better, but you can't complain being 18 under,' said Mackenzie. 'At the beginning of the day, I would have been pumped if someone told me I would be in a playoff.'
Darren Griff and Rob Johnson each had a chance to reach the playoff, but neither birdied the 72nd hole. Both posted rounds of 1-under 71 and tied for third place at minus-17.
Scott Gibson (68), Tom Stankowski (69), Will Yanagisawa (70) and Brian Guetz (73) tied for fifth place at 16-under-par 272.
Lee Curry and Matt Bettencourt both carded rounds of 3-under-par 69 on Sunday and shared ninth place at minus-15.