"Happy, but never satisfied," Mitchell said of his performance.
"Complacency is key — to make sure that you're never being complacent, you're always working to get better."
Mitchell, Calgary's third-stringer, got the start ahead of Drew Tate, who sat out with a forearm injury and Kevin Glenn, who's out with an elbow injury. The Texas native completed 29 of 33 pass attempts for 376 yards as the Stampeders advanced to 4-1 on the season.
And Rene Paredes did the rest with his sure foot.
The third-year Calgary kicker made all five of his field-goal attempts, setting a CFL record for consecutive regular-season field goals with 34, breaking Paul McCallum's record of 30 from 2011.
"I'm happy we got the win and I got the record at the same time," Paredes said, adding he's not predicting how many he might reach before the streak ends.
"That's 34. I'm going one by one. We'll see how it goes."
The Bombers dropped to 1-4 in front of 31,567 fans at Investors Group Field.
Winnipeg has lost eight straight games to the Stampeders, failing to secure a victory against the Calgary team since Week 2 of 2009.
Mitchell threw TD passes to Greg Carr, Anthony Parker and Marquay McDaniel. Rob Maver added a 50-yard punt single and Paredes' field goals from 37, 19, 26, 32 and 23 yards sealed the deal.
The game marked the first CFL matchup in 17 years to feature quarterbacks making their first pro starts for both teams as Bombers' backup Justin Goltz took the field for Buck Pierce, who suffered a stomach injury in last week's loss to Toronto.
Goltz threw a pair of touchdown passes to third-year receiver Jade Etienne from five and eight yards out, marking Etienne's first and second career CFL TDs.
Bomber Justin Palardy connected on a 41-yard field goal and Chad Simpson ran in a two-yard TD with 1:19 left in the game.
Calgary led 8-7 after the first half, 17-14 at halftime and 27-14 after the third quarter.
"I don't think they came out and really did anything different in the second half," said Goltz, who was 19-of-35 for 194 yards, two TDs and one interception.
"I think we have to take a lot of that on our own shoulders. There was a handful of throws I would have liked to have back and there were throws I know I can complete."
Pierce said late this week he felt healthy enough to play, but head coach Tim Burke went with Goltz, saying the trainers told him otherwise.
Goltz has no idea if he did enough to earn a second start in B.C. on Aug. 5.
"I don't know. That's out of my control," he said. "I wish I would have done more and made it more concrete, to be completely honest."
Both quarterbacks notched their first career touchdown passes in the opening quarter.
Mitchell tossed a 27-yard TD strike to the six-foot-six Carr in the end zone, who outstretched five-foot-nine defensive back Marty Markett for the ball at 12:23.
Goltz recorded his first TD pass in a six-play, 73-yard drive that included his 40-yard pass to Rory Kohlert on a broken play. Calgary was also called for a face mask on the play and Winnipeg moved to Calgary's 11-yard line.
After a six-yard run by Simpson, the Michigan native fired a quick pass to Etienne as he crossed in front of the goal post with time running out in the opening quarter. Goltz sent Etienne his second TD pass on the same route at 3:38 of the second quarter.
"It's nice, but it sucks to lose," said Etienne, who hails from Regina.
After a turnover on downs by the Bombers, Paredes booted his fourth field goal of the game, improving Calgary's lead to 27-14 with less than three minutes left in the third quarter, and securing the victory for the Stamps as they head into their bye week.
The Bombers worked for Simpson's late TD run, which finished off a 17-play, 75-yard drive.
Calgary running back Jon Cornish rushed 16 times for 88 yards, lead both teams in receiving yards with six catches for 87 yards.
"Thanks to a great team effort we had a balanced offence," said Stamps head coach John Hufnagel, who added that Mitchell read and saw the field well. "First of all, our offensive line played great. They opened up some holes and they provided Bo some protection so my compliments really go to my offensive line."