Johnson attended Gilmer High School, before moving on to Stephen F. Austin State University, where he was a three-year starter at running back.

On July 28, 2015, he was signed to improve the team's depth, after running back Ryan Williams was released.

Because of injuries to top running backs Joseph Randle, Darren McFadden and Lance Dunbar, he was forced into taking the majority of the reps in practice and in games.

[4] He had a good preseason, starting the first and fourth games, finishing as the team's rushing leader (37 carries for 139 yards and 2 touchdowns) and earning the nickname "Gus the Bus".

[10] On August 29, 2016, the San Diego Chargers claimed Johnson off waivers to replace Branden Oliver, who tore his Achilles tendon in the third preseason game.