The center spent seven seasons with the Cleveland Browns before he signed a five-year, $45 million contract with the Falcons. He never experienced a winning campaign, but his level of play didn't come into question.
He developed into a premier center, made three Pro Bowls and earned a reputation as one of the league's smartest and toughest blockers.
In 2015, the Falcons featured four of the same starting offensive linemen as this season. Mack's addition is the only difference between that unit with an 8-8 record and this season's 11-5 Super Bowl squad.
Mike Person started 14 games last season, but he never quite grasped the responsibilities of a center. With Mack over the ball, the Falcons averaged 20.1 more yards per game on the ground and 41.7 through the air.
In short, he changed how Ryan and the scheme operated.
"We joke around with him about being a genius," Falcons fullback Patrick DiMarco told Bleacher Report. "He understands the offense and knows football at another level. He's making all of our calls along the line. That takes so much off of Matt.
"Over the last few years, Matt had to make the 'Mike' calls, tell the line what to do and called hot routes. When Mack first arrived, he said, 'Hey, Matt. You worry about the coverage. I got the protection.' It's shown and taken so much off of Ryan's shoulders. He can now be himself and attack defenses."
With fewer responsibilities, Ryan's game elevated. He set career highs with a 69.9 completion percentage, 4,944 passing yards, 9.26 yards per attempt, 38 touchdowns and a career-low seven interceptions. In the playoffs, he's averaged 365 passing yards per game and posted a 7-to-0 touchdown-to-interception ratio.
Against the Seattle Seahawks' famed Legion of Boom defense in the NFC Divisional Round, Mack excelled, as EA Madden NFL Mobile noted:
EA Madden NFL Mobile @EAMaddenMobile
FBO @alexmack55 – allowed 0 sacks and helped block for 102 rushing yards against the no. 2 run defense in last week’s win over Seattle. https://t.co/ULwfkIgoiF2017-1-20 18:30:08
In the run game, Mack sets the tone in offensive coordinator Kyle Shanahan's zone-heavy scheme—a system he learned in Cleveland in 2014. Mack establishes the point of attack with his ability to reach defensive tackles and linebackers. His athleticism creates more space for running backs, as he's able to consistently widen lanes and reach linebackers on deuce (combo) blocks.
His attitude is infectious too.
"One of the coolest things about him is watching him finish," left tackle Jake Matthews said, per The Ringer's Robert Mays. "He never takes a play off. It's impressive to see him go downfield and finish guys."
NFL Network's Brian Baldinger provided an example from the NFC Championship Game against Green Bay Packers nose tackle Letroy Guion:
Brian Baldinger @BaldyNFL
Behind every good @AtlantaFalcons run there is a great finish by @alexmack55 https://t.co/hCzJ7K9GHN2017-1-31 00:49:20
In the above video, Mack maintained his ground off the snap and flipped his hips to hook Guion as right guard Chris Chester helped with the deuce block. He then moved to the second level and finished the play with authority.
"I think offensive line play is really important, and you really set your team up to have success," Mack told ESPN.com's Mike Triplett. "It takes everybody. It's not just one person making a play."
Mack's presence extends beyond the field; he makes his linemates better in the film room too.
"I've said since the day he got here, just learning from the way he prepares and the way he breaks down defenses has been really big for myself and growing as a player," Matthews said, per Triplett. "He's always on top of everything, so it's real impressive to watch him go out and do what he does. He fights through everything, doesn't complain about anything. So he's been a real good leader for this team."
An offensive line is only as strong as its weakest link. Five men must work in cohesion on every snap.
The center holds everything together. The position may not be valued as highly as the game's top left tackles or even some of the guards. Yet a player of Mack's caliber can bring the unit together thanks to his combination of intelligence, attitude and physical prowess.
"I think you can kind of see [Mack's influence] this year with the transformation Atlanta's gone through," former Browns teammate and current Kansas City Chiefs right tackle Mitchell Schwartz said, per Mays. "Obviously, Julio [Jones] is playing amazing, and Matt Ryan's having a career year, but I think that's all kind of linked to their offensive line stability and [Alex] going there and stabilizing it, just like he did for us [on the Browns]."
Mack's injured ankle is a major concern as the Falcons prepare for the Patriots. According to Pro Football Talk's Josh Alper, he has been limited in practice, but he's expected to play.
"I'm feeling good," Mack said. "I try not to talk too much about injuries right now. I think we're going to be smart, and I think that we'll be ready for game day."
The last time a center's presence was this important to the Super Bowl, Barret Robbins took an ill-advised trip to Tijuana, Mexico.
The Patriots are massive along their defensive interior with Alan Branch, Vincent Valentine and Malcom Brown (all three are listed at 320-plus pounds). They control blockers at the point of attack and haven't allowed a 100-yard rusher all season.
It falls on the Falcons offensive line to get these space-eaters moving laterally and create lanes for Devonta Freeman and Tevin Coleman. Shanahan's offense is predicated on being able to run the football, which sets up the play-action passing attack.
Matthews, Chester, Andy Levitre and Ryan Schraeder form an outstanding group of blockers, but Mack's addition helped them morph into something more.
If the Falcons win Sunday, Ryan will be showered with praise. Even so, Super Bowl LI will be won in the trenches, and the root of the Falcons' success starts with Mack.