A native of Stockton, California, Johnson attended Edison High School, where he played football and was an unranked wide receiver prospect.

[8] He competed with Bradley Fletcher, Jerome Murphy, Josh Gordy, Quinton Pointer, and Kendric Burney throughout training camp for the St. Louis Rams' third cornerback position.

[9] Johnson was named the Rams' third cornerback on their depth chart to begin the season, behind fellow rookie Janoris Jenkins and veteran Cortland Finnegan.

On September 30, Johnson recorded a tackle, one pass deflection, and his first career interception, picking off Seattle Seahawks quarterback Russell Wilson during a 19-13 victory.

[10] On November 11, Johnson made his first career start and collected four solo tackles as the Rams had a 24-24 overtime draw with the San Francisco 49ers.

[11] He finished his rookie season with a total of 31 combined tackles (29 solo), eight pass break ups, and two interceptions in three starts and 16 games.

[13] In the Rams' 27–24 season-opening victory over the Arizona Cardinals, Johnson made three combined tackles, broke up two passes, and intercepted quarterback Carson Palmer, returning it 29 yards to the four-yard line.

[15] On December 15, Johnson picked off quarterback Drew Brees and recorded six tackles in a win over the New Orleans Saints, marking his fifth interception of his career.

On August 23, 2014, Johnson made a solo tackle before exiting the Rams' 33–14 victory at the Cleveland Browns and was carted off the field in the second quarter of their third preseason game.

[19] In Week 13, Johnson recorded six solo tackles, broke up two passes, made two interceptions, and a touchdown as the Rams routed the Oakland Raiders 52–0.

[22] Head coach Jeff Fisher retained Johnson and Jenkins as the starting cornerback duo to begin the 2015 regular season.

He started in the St. Louis Rams' season-opener against the Seattle Seahawks and recorded three solo tackles, broke up a pass, and intercepted Russell Wilson during a 34–31 victory.

The following week, he collected a season-high ten solo tackles and broke up a pass in the Rams' 19–16 loss at the San Francisco 49ers.

[28] On September 25, 2016, Johnson collected a season-high nine combined tackles and two pass deflections during a 37–32 victory at the Tampa Bay Buccaneers in Week 3.

[29] On October 9, 2016, he made two solo tackles and broke up a pass before being carted off the field in the second quarter of the Rams' 30–19 loss against the Buffalo Bills with an ankle injury.

[33] In Week 13, Johnson recorded a season-high seven solo tackles and broke up a pass in the Rams' 32–16 victory at the Arizona Cardinals.