Considered a three-star recruit by Rivals.com, Barner was listed as the No. 38 running back in the nation in 2008.

In the 2009 season, Barner had 61 carries for 366 yards and three rushing touchdowns to go along with 41 kick returns for 1,020 net yards and a 100-yard kick return touchdown, which came against the UCLA Bruins.

In the 2010 season, Barner’s role on the offense expanded. In the season opener against New Mexico, he had 17 carries for 147 rushing yards and four rushing touchdowns to go along with a 60-yard touchdown reception. In the next game, a victory over Tennessee, he had a punt return touchdown. He suffered a career-threatening concussion after a helmet-to-helmet hit against Washington State on October 9. In the annual rivalry game against Oregon State, he had 15 carries for 133 rushing yards and a touchdown. Overall, he had 551 rushing yards, six rushing touchdowns, 121 receiving yards, two receiving touchdowns, 13 kick returns, 236 net kick return yards, 10 punt returns, 132 net punt return yards, and a punt return touchdown in the 2010 season.

In the 2011 season, Barner was the second leading rusher in the Ducks’ backfield. On September 24, against Arizona, he had 72 rushing yards, two rushing touchdowns, and two receptions for six yards and a receiving touchdown. On October 15, against Arizona State, he had 31 carries for 171 rushing yards and a touchdown. He followed that up with 115 rushing yards and two rushing touchdowns in a victory over Colorado. On November 19, against Southern California, he had 123 rushing yards and two rushing touchdowns. Overall, in the 2011 season, he had 939 rushing yards, 11 rushing touchdowns, 17 receptions, 184 rushing yards, and three rushing touchdowns.

The Ducks won the 2013 Fiesta Bowl over Kansas State with Barner having 143 rushing yards and a 24-yard touchdown reception. Following his senior season, Barner was named to the AP All-American second team and the All-Pac-12 Conference First Team. He finished in ninth place in the 2012 Heisman Trophy voting.

Barner was born in Lynwood, California to parents Gary and Wilhelmenia Barner. He has five older brothers and one older sister. He graduated with a degree in criminology in spring 2012. Kenjon has two sons born in 2013 and 2017.

On April 27, 2013, Barner was drafted by the Carolina Panthers in the sixth round (182nd overall) of the 2013 NFL Draft. In his rookie year, Barner played a limited role on special teams. He had six carries for seven yards and two catches for seven yards.

On August 19, 2014, Barner was traded to the Philadelphia Eagles for a conditional seventh round pick in 2015. The trade reunited Barner with his former head coach at University of Oregon, Chip Kelly. He was released on August 30, 2014. On November 4, 2014, he was signed to the Philadelphia Eagles’ practice squad.

During the 2015 preseason, Barner attempted to make the roster behind three former NFL starters, DeMarco Murray, Ryan Mathews and Darren Sproles. Looking to make an impact in the return game, Barner found near immediate success. In the first preseason game against the Indianapolis Colts he returned a punt 92 yards for a touchdown to end the second quarter. In the second preseason game against the Baltimore Ravens, he returned another punt for a 68-yard touchdown. He made the roster and ended up with 124 rushing yards and 22 receiving yards in the 2015 season.

Barner played in 13 games in 2016, rushing 27 times for 129 yards and two touchdowns and recorded five receptions for 42 yards while contributing on special teams as a returner. He was placed on injured reserve on December 20, 2016.

On March 20, 2017, Barner signed a one-year contract with the Los Angeles Chargers. He was released on September 2, 2017.

On September 26, 2017, Barner signed a one-year deal with the Philadelphia Eagles after an injury to running back Darren Sproles. In Week 5, Barner averaged 36 yards on three punt returns, including a 76-yarder, in a 34-7 win over the Cardinals, earning him NFC Special Teams Player of the Week. He finished the regular season with 57 rushing yards, one rushing touchdown, 56 receiving yards, 10 kick returns for 194 net yards, and 27 punt returns for 240 net yards. The Eagles finished the season 13-3, clinching the NFC East division. The team reached Super Bowl LII, where they defeated the New England Patriots 41-33, winning their first Super Bowl. In the game, Barner had three kick returns for 73 net yards.

Barner’s cousin Melvin Carter was elected mayor of St. Paul, Minnesota in November 2017.

On May 11, 2018, Barner signed with the Carolina Panthers. He was released on August 31, 2018, as part of final roster cuts.

On September 11, 2018, Barner signed with the New England Patriots. On September 19, 2018, the Patriots released Barner, but re-signed him a week later after Rex Burkhead was placed on injured reserve. He was released again on October 4, 2018 before Barner was re-signed four days later. On November 13, 2018, the Patriots released Barner.

On November 14, 2018, Barner was claimed off waivers by the Carolina Panthers. He was placed on injured reserve on December 28, 2018. He had 19 carries for 71 yards on the 2018 season.

On March 14, 2019, Barner signed a one-year contract with the Atlanta Falcons. In Week 11 of the 2019 season, against the Carolina Panthers, he had a 78-yard punt return for a touchdown in the 29–3 victory, earning NFC Special Teams Player of the Week honors.

On August 12, 2020, Barner signed a one-year contract with the Baltimore Ravens. He was released during final roster cuts on September 5, 2020.

On September 10, 2020, Barner was signed to the Tampa Bay Buccaneers practice squad. He was elevated to the active roster for the team’s weeks 3 and 4 games against the Denver Broncos and Los Angeles Chargers, respectively, and reverted to the practice squad after each game. He was suspended by the NFL for four games after violating the league’s performance-enhancing drugs policy on October 6, 2020.