“It was just grit,” Derrick Henry, the Tennessee running back, said of the comeback. “You’ve got to believe in each other.”

The game was not particularly impressive statistically for Mariota, who completed 19 of 31 passes for 205 yards, two touchdowns and one interception, but in the end he did all of the little things necessary to extend drives and will his team to victory. Mariota spread the ball around to eight receivers — including himself — and deftly avoided Kansas City’s pass rush, aside from two sacks.

Tennessee appeared to come into the game at a disadvantage because its starting running back, DeMarco Murray, was out with a knee injury. But the Titans switched seamlessly to Henry, who gained 156 yards on 23 carries, including a 35-yard touchdown run.

The collapse of the Chiefs, who had gone into halftime with a 21-3 lead, somewhat mirrored their regular season, which began with a 5-0 start but also included a stretch of six losses in seven games.

Alex Smith completed 24 of 33 passes for 264 yards and two touchdowns, spending the first half carving up Tennessee with a receiving combination of Tyreek Hill, Travis Kelce and Demarcus Robinson. But Kelce had to sit out the second half after sustaining a concussion in the second quarter, and the Chiefs never recovered.

Even Kareem Hunt, Kansas City’s dynamic rookie running back, who had come out of nowhere to lead the N.F.L. in rushing this season, seemed unable to do anything in the second half. He finished the game with 11 carries for 42 yards and a touchdown.