The rookie quarterback strode into the locker-room at training camp back in 2012 with the confidence of a 10-year veteran, not a player looking for a job.
"He's probably one of the most arrogant rookies to come in," Simpson recalled with a smile.
Brash young players can sometimes rub veterans the wrong way, but Mitchell gained the trust of teammates early and in three seasons has gone from a third-string pivot to the starter on a 15-3 club that will meet the Hamilton Tiger-Cats in Sunday's 102nd Grey Cup.
"He applies himself. I think that's where his confidence comes from," added Simpson. "It's not just because he has a strong arm.
"Maybe that's how he originally got his confidence, but he definitely prepares himself very well and he works hard. He goes out there and performs ... I'm a fan."
Stampeders running back Jon Cornish, the CFL's most outstanding player in 2013, remembers Mitchell turning heads almost immediately.
"So here's this guy coming in throwing in scout team and just dropping bombs on our first-team defence," said Cornish. "I think that allowed him to sort of understand that he can play at this level.
"It just speaks to how great his understanding of this game is and how much effort he puts into becoming a better player."
A native of Katy, Texas, Mitchell completed 63.3 per cent of his passes in 2014, totalling 3,389 yards and 22 touchdowns against eight interceptions for the CFL's most potent offence.
"He's got the good little Texas quarterback swagger about him," said Calgary running back Rob Cote. "It's good. A quarterback needs confidence and he's got that for sure. It's an advantage to have an even-keel guy. You don't want a big roller-coaster emotionally and he's a rock for our offence."
Mitchell was cool under pressure in his first playoff start last weekend in Calgary's victory in the West Division final victory over the Edmonton Eskimos, something he said has always been part of his makeup.
"I get excited when my helmet's on and I get hit or when we make a big play. That's when it comes out of me," said the 24-year-old. "As a quarterback you've got to keep calm. You've got to be thinking about the plays and not getting too overly excited about stuff. I've won one at every level and I'm very excited to get the opportunity to do it again, but I know it's tough.
"I know that it comes from having those teammates around you and having a full team effort in order to get a win."
While some players need to psyche themselves up for big games, Mitchell takes a more measured approach to his craft.
"When you play the game for so long it's hard to make too much of it," said the product of Eastern Washington University. "A lot of guys, they need the nerves. It's something that gets them ready.
"For me it's my job, it's my career, something that I love to do and I'm passionate about. Every opportunity and chance I get to go out there and play, I'm going to take advantage of it."
Mitchell took over the starting job this season from Drew Tate, who in turn beat out Kevin Glenn in 2013, and should give the Stampeders a steady hand moving forward.
"I know that every person in the locker-room understands he's the quarterback that we've been looking for," said Cornish. "It's just nice to have that consistency, the leadership and the level-headedness under centre."
Stampeders head coach and general manager John Hufnagel said Mitchell has done everything he's asked of him since he came to the CFL three seasons ago.
"The game's not too big for Bo," said Hufnagel. "He manages it, he makes plays. He'll make mistakes like everybody, but he'll come back and manage the game."
Mitchell was the No. 3 quarterback when the Stampeders lost the 2012 Grey Cup game to the Toronto Argonauts and was the backup when the team dropped last year's West final to the Saskatchewan Roughriders.
"I'm a teammate first. Being third-string quarterback, second-string or first-string, you feel the pain of your brothers and teammates whenever you lose that big game because we all put in work," he said. "We all put in the sweat and the tears to get there."
Mitchell will be opposite Hamilton's Zach Collaros on Sunday, another quarterback making just his second playoff start.
"Zach is a very competitive quarterback and that's what I love about him," said Mitchell. "I love watching him play and the way he fires his team up. We are similar in that fact.
"He's done a great job coming back (from injury) and leading his team to the Grey Cup."
While he doesn't do a lot of talking in the locker-room, Mitchell is happy to shoulder the expectations put on his club as it looks for its first title since 2008.
"I like fielding the questions when they question me because that means they're not questioning other people," he said. "I don't mind them putting the pressure on me because I prepare the way I do to make sure that when any situations comes, I can step up and handle it."