He attended Northwestern High School in Hyattsville, Maryland and played wide receiver and safety for head coach Brian Pierre.

Johnson received ten offers in total from Boston College, Lehigh, Bucknell, Buffalo, Duke, UMass, Missouri, Pittsburgh, Towson, and Ohio.

On July 23, 2012, he officially committed to play for Boston College and cited academics as the main reason on why he chose to attend BC.

On March 14, 2017, he participated at Boston College's pro day and chose to attempt the broad jump and run positional drills.

Johnson recorded three combined tackles, defended two passes, and returned the first interception of his career for 69-yards after picking off Seattle Seahawks quarterback Russell Wilson in a 10–16 loss.

[15] On December 10, 2017, Johnson collected a season-high 11 combined tackles (seven solo) and two pass deflections during a 43–35 loss to the Philadelphia Eagles in Week 14.

[16] In Week 1 against the Oakland Raiders, Johnson intercepted quarterback Derek Carr and made 9 tackles on Monday Night Football.

In the NFC Championship Game against the New Orleans Saints, Johnson had 4 tackles and caught the big interception in overtime to help the Rams win 26–23 to send them to Super Bowl LIII where they faced the New England Patriots.

[19] In Week 3 against the Cleveland Browns, Johnson intercepted Baker Mayfield in the end zone late in the fourth quarter, sealing a 20–13 Rams win.

[21] In Week 3 against the Buffalo Bills, with the Rams down 28–3 in the third quarter, Johnson was credited with an interception after coming down with simultaneous possession of the ball with Bills tight end Tyler Kroft, allowing the Rams to rally and take the lead in the fourth quarter before Buffalo responded with a game-winning drive in the last few minutes.