Johnson opted to redshirt his first year at Rutgers University to become stronger and work on his game. As a freshman, he averaged 4.8 points and 5.7 rebounds per game,[6] which ranked third among Big Ten freshman.[7] He posted 18 points and 14 rebounds, both career-highs in a 63–44 victory against Lafayette on December 22, 2019.[8] On January 3, 2020, he scored 18 points and grabbed 14 rebounds in a 79–62 win against Nebraska.[9] In his sophomore season, Johnson entered the starting lineup and averaged 7.8 points, 7.9 rebounds and 1.5 blocks per game. He shot 60.2 percent from the field, which led the Big Ten.[10] As a junior, Johnson averaged eight points, 8.5 rebounds and 2.4 blocks per game,[11] and the Scarlet Knights were selected for the 2021 NCAA tournament.[12] He earned Big Ten All-Defensive Team honors.[13] He completed his undergraduate degree in electrical and computer engineering in May 2021.[14][1]
In 2021–22, Johnson transferred to the University of California, Los Angeles, choosing the Bruins' program over Stanford's.[15] He filled in for starting forward Cody Riley, who suffered a knee injury in the season opener, which kept him out for almost two months.[16] Johnson played in all 35 games, starting in 15, and averaged 3.6 points, 5.4 rebounds and 1.3 blocks per game.[14] He was named to the Pac-12 All-Defensive Team.[17] After the season, Johnson announced that he would not return for another season and instead planned to focus on completing his master's degree and pursuing a career in engineering.[14][18] He considered a pro basketball career overseas, but decided against living in a country where he did not speak the language.[16]