Tagovailoa entered the game at halftime, replacing a struggling Jalen Hurts, and threw three touchdown passes, including the game-ender to give the Crimson Tide its fifth national championship since 2009 under coach Nick Saban.
After Alabama kicker Andy Pappanastos missed a 36-yard field goal that would have won it for the Tide (13-1) in the final seconds of regulation, Georgia (13-2) took the lead with a 51-yard field goal from Rodrigo Blankenship in overtime.
Tagovailoa took a terrible sack on Alabama's first play of overtime, losing 16 yards. On the next play he found Smith, another freshman, streaking down the sideline and hit him in stride for the national championship.
This game will be remembered for Saban's decision to change quarterbacks trailing 13-0.
"I just thought we had to throw the ball, and I felt he could do it better, and he did," Saban said. "He did a good job, made some plays in the passing game. Just a great win. I'm so happy for Alabama fans. Great for our players. Unbelievable."
What was he thinking as the winning pass soared?
"I could not believe it," Saban said. "There's lots of highs and lows. Last year we lost on the last play of the game and this year we won on the last play of the game. These kids really responded the right way. We said last year, 'Don't waste the feeling.' They sure didn't, the way they played tonight."
Saban now has six major poll national championships, including one at LSU, matching the record set by the man who led Alabama's last dynasty, coach Paul Bear Bryant.
This one was nothing like the others.
The all-Southeastern Conference matchup was all Georgia in the first half before Saban pulled Hurts and went with the five-star recruit from Hawaii to start the second half.
The Tide trailed 13-0 at halftime and 20-7 in the third quarter after Georgia's freshman quarterback, Jake Fromm, hit Mecole Hardman for an 80-yard touchdown pass that had the Georgia fans feeling good about ending a national title drought that dates back to 1980.
A little less than a year after the Atlanta Falcons blew a 25-point lead and lost in overtime to the New England Patriots in the Super Bowl, there was more pain for many of the local fans. With the title game being held 70 miles from Georgia's campus in Athens, Dawg fans packed Mercedes-Benz Stadium, but it turned out to be sweet home for Alabama.
It was not without angst.
Alabama drove into the red zone in the final minute and Saban started playing for a field goal that would end the game and win it for the Tide. A nervous quiet gripped the crowd of 77,430 as 'Bama burned the clock. With the ball centered in the middle of the field, Pappanastos lined up for a kick to win the national championship. The snap and hold looked fine, but the kicked missed badly to the left.
For the second straight week, Georgia was going to overtime. The Bulldogs beat Oklahoma in a wild Rose Bowl in double overtime to get here, and after Jonathan Ledbetter and Davin Bellamy sacked Tagovailoa for a big loss on the first play, Alabama was in trouble - second-and-26.
Not for long. Tagovailoa looked off the safety threw the biggest touchdown pass in the history of Alabama football.