He began his college football career with the UC Davis Aggies before transferring to the UCLA Bruins program.

[2][1] His mother Jacqueline, a speech pathologist at an elementary school,[3] raised him and his older brother Daniel as a single parent.

[7] As a senior, he earned first-team all-conference honors in the Golden League, and was named the conference's running back of the year.

232 running back in the class of 2015,[3] Kelley began his career in the Football Championship Subdivision (FCS) at the University of California, Davis,[8] the only school that pursued him.

[6] Kelley signed as a walk-on with the Bruins of the Football Bowl Subdivision (FBS), and served on the scout team in 2017 while sitting out the year as a redshirt due to transfer rules.

[6][8][9] Leading up to the 2018 season, he encountered another coaching change when UCLA replaced Jim L. Mora with Chip Kelly.

[1][12] Following an open week in UCLA's schedule, he regrouped and ran for 124 yards in 12 carries against Colorado after being the team's best player in training sessions leading up to the game.

[14] On the road in Week 6 against California, UCLA earned their first win of the season as Kelley ran for 157 yards and three touchdowns in a 37–7 victory.

[15] Against USC, Kelley had a career-high 289 yards rushing on 40 carries to help the Bruins earn their first win in four years in their crosstown rivalry with the Trojans.

[24] He rushed for 126 yards and two touchdowns the following week against Colorado, helping the Bruins earn their fourth win and surpass their total from a year before.

[28] After being named to the Senior Bowl roster as a replacement for Moss,[29] he improved his draft stock with a game-high 105 yards on 15 carries.

[35] In his NFL debut in the 2020 season opener, Kelley had 60 rushing yards on 12 carries and scored the team's only touchdown on a five-yard run in the fourth quarter in a 16–13 win over the Cincinnati Bengals.

[38] However, he struggled after fumbling in consecutive weeks in close games against the Carolina Panthers and Tampa Bay Buccaneers, which damaged his confidence and led to a drop in playing time.