Johnson’s father Johnnie also played at Texas and played for ten years in the NFL; he was inducted into the College Football Hall of Fame in 2007. Collin’s older brother Kirk is also a football player at Texas, and his sister Camille is a member of the track and field team. On June 26, 2020, Collin proposed to his girlfriend since high school, Sara Machado.

Attending Valley Christian in San Jose, Johnson was limited by injury in both his sophomore and senior seasons but still eclipsed 1,100 yards as a senior. By the winter of his sophomore year, Johnson had garnered scholarship offers from USC, Arizona State, Utah, Rutgers, Cal, and TCU, but committed to Texas on April 17, 2014 along with his brother Kirk, a running back who is a year older than Collin.

Johnson’s acrobatics and 124 yards against TCU in week four earned him National Player of the Week accolades. After a slight injury to his knee, Johnson missed the Nov 10 game against Texas Tech. In the 2018 Big 12 Championship Game, Johnson set the title game record for receiving yards with 177. The performance came after Johnson was excluded from the All-Big 12 team for 2018. His coaches also lauded him for increased blocking skills throughout the season.

Johnson was selected by the Jacksonville Jaguars in the fifth round with the 165th overall pick in the 2020 NFL Draft. The Jaguars received the 165th pick used to select Johnson as a result from a trade that sent Dante Fowler Jr. to the Los Angeles Rams.