Making his first CFL start, the B.C. Lions quarterback threw two second-half touchdowns and helped set up Paul McCallum's game-winning field goal with no time on the clock as the Lions took over second place in the West Division with a 24-22 win over the Saskatchewan Roughriders.
DeMarco was pulled from the game midway through the second quarter in favour of Buck Pierce but returned to start the second half.
The win was the second straight for B.C. (8-4) while Saskatchewan (8-4) lost its third in a row.
"I have a lot of great people on the sidelines right now," said DeMarco, who started in place of the injured Travis Lulay. "Travis was very calm. Buck was very calm. Joey (Elliott) helped me out as well.
"The biggest thing is just to keep you calm and help you with seeing the field. They really helped me out today and allowed me to talk through things and we got the victory at the end."
Pierce replaced DeMarco briefly after DeMarco's first 13 pass attempts yielded just 52 yards and five straight two-and-outs.
DeMarco had started the game with five straight completions.
But he made up for a shaky first half with the Lions trailing 12-6 early in the third quarter when he hit Nick Moore with a 43-yard touchdown strike.
The Lions got the ball right back when Saskatchewan quarterback Darian Durant fumbled for a second time.
DeMarco threw a 20-yard TD to Shawn Gore to make it 20-12 and McCallum then kicked an 84-yard kickoff single to extend the lead to nine.
"He's just a very well-composed individual," McCallum said. "Playing his first start, going in there, getting pulled, going back in and making things happen, that shows a lot about his character."
Durant threw an eight-yard touchdown to Weston Dressler with 38 seconds left to make it 22-21 as the Roughriders appeared poised to overcome two costly fumbles and snap their losing streak.
But Marco Iannuzzi returned the kickoff 28 yards and DeMarco passed for a pair of key first downs to set up McCallum's winning kick from 42 yards out.
"Paul's amazing," said DeMarco. "You don't even have to say anything to him. He gets in, he looks like he's bored out there, he turns his chinstrap, kicks the ball through, doesn't even celebrate with me and I'm jumping on his head.
"He's done this for a long time and there's no doubt in our mind he is our guy."
DeMarco finished 18-of-33 passes for 208 yards while Pierce went 2 for 3 for 16 yards.
Durant replied with 35-for-52 passing for 326 yards.
The Roughriders appeared to have the game won after Dressler's touchdown but, for the third straight game, ultimately paid the price for their turnovers and undisciplined ways.
Durant — who battled a bruised finger on his throwing hand — also fumbled on the game's second play from scrimmage, giving B.C. the ball at the Riders' 46-yard line.
That set up a 12-yard field goal from McCallum.
In the second quarter, with DeMarco on the sidelines, the oft-injured Pierce drew a roughing-the-passer penalty on his first series to move B.C. into scoring position and McCallum hit his second field goal of the game, from 32 yards, to make it 6-2.
A second roughing-the-passer penalty against Saskatchewan followed Durant's second fumble and put the Lions in scoring position for Gore's touchdown.
Saskatchewan had its own issues on offence in the first half as well, scoring only two punt singles despite starting six of its first eight possessions from its own 43 or better.
"Those guys are good, especially that secondary," said Riders slotback Geroy Simon, who played his former team for the first time after 12 seasons in B.C. and tied Ben Cahoon's all-time receptions mark in the fourth quarter.
"They're a veteran group, they really know how to make plays and they know how to hold in tough situations."
The Riders took their first lead of the game when Durant hit Chris Garrett — who started in place of injured running back Kory Sheets — with a 37-yard pass to the front corner of the end zone with two seconds to play in the half.
Chris Milo extended the lead to 12-6 when he hit a 15-yard field goal on Saskatchewan's first drive of the third quarter.
After the Riders fell behind again, Milo made 21-15 by hitting a 47-yarder after previously missing two in the game by hitting them off the right goal post.
"We've got to get back to playing good football," said Saskatchewan head coach Corey Chamblin. "Regardless of all the freakish things that may have happened we still have to be able to play football."