But he says he'll be siding with the westerners when the two teams meet again in Toronto on Sunday.
Harper was a 12-year-old growing up in Toronto when the two teams first clashed for the Cup in Vancouver, with the Argonauts looking for their first Grey Cup in almost 20 years.
The Argos were on the Calgary 11-yard-line with less than three minutes left to play when star running back Leon McQuay slipped on the rain-slicked turf and fumbled.
Calgary recovered, couldn't make a first down and punted, but the Toronto receiver accidentally kicked the ball out of bounds, turning it over to the Stampeders to run out the clock.
The McQuay fumble defined the game and the 14-11 Stampeder victory and left young Harper crying.
"I think that was the only time that I cried in front of the TV at a sports event," Harper said Monday as he answered questions at a Canada-U.S. business forum.
"We had been waiting for 20 years. I was growing up in Toronto. Obviously the Argonauts were my team."
Harper said, however, that has changed.
"I still have a soft spot for them, but now the shoe's on the other foot," he said.
"I now live in Calgary. I represent Calgary in the House of Commons so I've got to be on the side of my new home town. But I'll be happy with either."
He did note there's a silver lining to any outcome.
"The good thing about a Grey Cup is that a Canadian team always wins."