The score was tied 21-21 with 3 minutes remaining in the fourth quarter and the Rams facing third-and-7 at their own 24-yard line. Clemens was sacked by Jurrell Casey and fumbled, with Derrick Morgan recovering the loose ball.
On the next play, Johnson ran around the right end for a go-ahead, 19-yard touchdown. He finished with 150 rushing yards and two TDs on 23 carries.
Jake Locker went 13-of-22 through the air for 185 yards with two interceptions and ran for a fourth-quarter touchdown for the Titans (4-4), ended a three-game losing streak.
Clemens was limited to 210 yards and a touchdown on 20-of-35 efficiency for the Rams (3-6), who dropped a third straight. Zac Stacy led the St. Louis ground game, rolling up 127 yards and two touchdowns on 27 carries.