The CFL's leading rusher kept adding to his gaudy numbers this season and scored the winning touchdown with just over two minutes to play.
The New Westminster, B.C., native plowed three yards into the end zone for the last score of a heated, see-saw battle between the two teams with the best records in the CFL.
But Calgary (14-3) secured first place in the CFL's West Division with a game remaining for the right to host the division final Nov. 17.
"That's why you work so hard throughout the regular season and when you get close you want to clinch the deal," Stampeder head coach John Hufnagel said. "This wasn't a must-win situation for us, but we did want to win it in front of our home fans."
The Roughriders (11-6) get the division semifinal Nov. 10 against the B.C. Lions. The winner advances to face the Stampeders at McMahon with the division winner earning a berth Nov. 24 in the Grey Cup in Regina.
With Calgary trailing 15-10 after the first half because of turnovers, Hufnagel replaced starting quarterback Kevin Glenn with Drew Tate, who completed 11-of-16 passes for 181 yards.
Tate also ran for a touchdown and his 37-yard pass to wide receiver Jeff Fuller in the fourth quarter was the play preceding Cornish's TD.
"We were having a little bit of problems with our offensive line," Hufnagel said of his halftime decision. "I had guys who weren't 100 per cent healthy after the first couple of series and we wanted more mobility at that position."
Calgary concludes the regular season on the road against the Lions on Friday, while the Roughriders are at home to the Eskimos.
With Tate, who spent much of this season managing an arm injury, back in the mix, it renews the question of which quarterback will start in the post-season. Hufnagel said following the game it is possible Glenn, Tate and Bo Levi Mitchell will all take snaps against the Lions.
Cornish's 109 yards padded his own single-season rushing record for a Canadian running back to 1,721 this season. Saskatchewan's Kory Sheets, the league's No. 2 rusher behind Cornish, had 42 yards on 11 carries.
Glenn and Cornish each fumbled the ball twice in the first half. Glenn completed 11-of-17 passes for 120 yards and threw a touchdown pass to Anthony Parker before he was replaced by Tate.
Stampeder kicker Rene Paredes made his three field-goal attempts in front of an announced 33,671 at McMahon Stadium.
Darian Durant threw a touchdown pass to Taj Smith that gave Saskatchewan a temporary lead in the fourth quarter.
The veteran quarterback also ran for a TD for Saskatchewan in the game. Roughrider kicker Chris Milo was 4 for 4 in field-goal attempts.
"We squandered it," Durant said. "The defence gave us plenty of opportunities to put some points on the board.
"We’re not going to win kicking field goals. It's always a dog fight when we play these guys and tonight was no different."
Sheets and Calgary defensive end Charleston Hughes had traded social media barbs via Twitter heading into the game because Sheets had declared himself that best back in the league.
Hughes collected three and Cordarro Law two of Calgary's seven sacks in the game, while the 'Riders registered three.
Hughes took over the league lead in sacks with 18. Saskatchewan's Alex Hall didn't register one in the game to remain at 15 and one ahead of Law.
"There was a lot of talk throughout the week," Hughes said. "I wanted to back it up.
"It was something like a statement game. For one, that I am the best defensive end in this league and for two, the second-best defensive end in this league is right next to me. Cordarro Law."
Roughrider defensive back Dwight Anderson, a Stampeder for three seasons before heading to Saskatchewan, had a direct message for his former team.
"We're the better the team anyway," Anderson said. "Calgary sucks."
The Roughriders are under considerable pressure to participate in the Grey Cup hosted by their province.
A win Saturday would have extended the race for first place in the division to the final week of the regular season, although a Stampeder victory over the Lions on Friday would have put it out of Saskatchewan's reach.
Saturday's game may have been a preview of the division final, which would pit the top offences and defences in the league against each other again.
"Saskatchewan is a great team, great defensive line, great linebackers, great offence," Cornish said. "It's one of those things you know you're going to have to play them again and I think we're a little bit better prepared now that we've played them simply because we know we can't make mistakes.
"I was just trying to do my part for the team's win. There was a lot of hype and build up to this game. Maybe it affected me a little bit. I think that's the first time the ball's been falling out of my hand."
Notes: With 84 receiving yards, Calgary receiver Marquay McDaniel reached a season 1,000 for the first time in his career . . . Saskatchewan lost its seventh in a row at McMahon and are 3-10-1 there since 2005.