Joseph Johnson was a three-year varsity player for Gretna High School, playing 35 games for head coach Dave Shrader. The 6-3, 235-pound linebacker recorded 288 total tackles during his prep career, including 102 solo stops. As a senior, he had three sacks, three interceptions and two fumble recoveries – one of which he returned for a touchdown. Johnson was named to the Lincoln Journal Star Super-State team in 2017. Following his prep career, Johnson represented the South Squad in the 2018 Nebraska Shrine Bowl. He was named the game's defensive MVP after totaling a team-high nine tackles, including four for loss with 2.0 sacks and a forced fumble.