A two-star recruit who was ranked the No. 232 running back in the class of 2015, Kelley began his career in the Football Championship Subdivision at the University of California, Davis, the only school that pursued him. With the Aggies, he coped with injuries and shared time with another fine back in Manusomo Luuga. In his first season, Kelley was a year younger than his fellow freshmen. He did not receive a full scholarship until the start of his sophomore year, but relinquished it and transferred after the season, when head coach Ron Gould and the rest of the coaching staff were fired. Although the new staff wanted him to stay, Kelley challenged himself to reach college football's highest level. He ended his UC Davis career with 1,139 yards and seven touchdowns in two seasons. As part of his release from the school, Kelley was permitted to transfer to just two schools in California: UCLA and USC. He also received interest from Boise State. Kelley signed as a walk-on with the Bruins of the Football Bowl Subdivision, and served on the scout team in 2017 while sitting out the year as a redshirt due to transfer rules. Leading up to the 2018 season, he encountered another coaching change when UCLA replaced Jim L. Mora with Chip Kelly. The new coach awarded a scholarship to Kelley. A redshirt junior, Kelley did not play much until Week 4 at Colorado. He was benched and did not play a week earlier against Fresno State after being outperformed during practice. In the first two weeks, he had just 11 carries for 27 yards. 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. Kelley became one of the nation's top running backs over the final two months of the season. He emerged as the Bruins' top offensive player with six 100-yard games, and UCLA's scoring increased as well. 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. 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. It was the most rushing yards for a player on either team in the history of the rivalry. He scored two touchdowns, including one on a career-long 61-yard run. Kelley ended the season with 1,243 yards rushing, the 10th-highest single-season total in UCLA history, and his 113 rushing yards per game ranked fourth-highest in the Pac-12 Conference and ninth in the nation. He ran for at least one touchdown in each of the final eight games, and finished with 12 touchdowns to rank second in the conference. Kelley earned honorable mention in All-Pac-12 voting by conference coaches, while the Associated Press named him to their first team. He was named the Bruins' most valuable player. In 2019, Kelley returned for his senior year after deciding against entering the NFL Draft. He injured his right knee in practice, and subsequently missed most of training camp. He did not play in the season opener against Cincinnati. On October 26 against Arizona State, Kelley ran 34 times for 164 yards and a career-high four touchdowns in a 42–32 upset over the No. 24 Sun Devils. 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. In the final game of his college career, Kelley ran for 76 yards against Cal to finsh the season with 1,060 yards, becoming the eighth player in UCLA history to run for 1,000 yards in consecutive seasons. His 96.4 yards rushing per game and 12 rushing touchdowns were both second in the conference behind Zack Moss of Utah. Kelley was named second-team All-Pac-12, the only Bruin to be named to either the first or second team. After being named to the Senior Bowl roster as a replacement for Moss, he improved his draft stock with a game-high 105 yards on 15 carries.
Professional career
Kelley was selected by the Los Angeles Chargers with the 112th pick in the fourth round of the 2020 NFL Draft.
