He grew up interested in music and film and began playing drums at age nine. He attended Felix Varela High School, where he joined the school’s drama society Troupe 6162 and played varsity football and wrestling. Stating that he was “in love with improv and comedy” he began improvisation lessons and participated in stage productions at the Roxy Performing Arts Center. In 2008, he became a member of “Impromedy”, a sketch comedy improv group based in Miami.

By 2011, he was cast in the short films Wurlitzer and The Truth In Being Right and the horror film Cousin Sarah. He also made a guest appearance on ABC Family’s sitcom Melissa & Joey as Miller Collins.

In 2012, he was selected through industry channels to audition as one of 15 contestants on the second season of The Glee Project, a television reality show for which the prize was a guaranteed seven-episode arc on the following season of Glee. He was one of three finalists and ultimately won. After appearing as Ryder Lynn on the fourth season of Glee, he was later nominated and won the Teen Choice Award for Choice TV Breakout Star. Jenner, along with co-stars Jacob Artist, Becca Tobin, Alex Newell, and Melissa Benoist, was promoted to series regular status for the fifth season.

Jenner became engaged to his former Glee co-star Melissa Benoist in 2013 and the pair reportedly married in 2015. However, that same year, Benoist commented that they had been married “longer than anybody knows,” with some sources claiming that the couple actually wed in 2013. Benoist filed for divorce in late December 2016, citing irreconcilable differences, and the divorce was finalized in December 2017.

In the summer of 2013, Jenner wrote the screenplay for Billy Boy. He and Benoist went on Kickstarter to ask fans to help fund the project’s production; as of December 11, 2013, the project has reached its $100,000 goal. Jenner was subsequently cast in the horror film Within. In late 2015, Jenner was cast in a recurring role as Adam Foster on Supergirl, playing opposite his then-wife Melissa Benoist, who stars as the series’ title character. His first appearance aired on January 25, 2016.

In 2016, Jenner starred as Jake Bradford in Richard Linklater’s comedy film Everybody Wants Some!!, which was released on March 30, and played Hailee Steinfeld’s character’s brother in the coming-of-age drama The Edge of Seventeen.

In April 2016, he was announced to have joined the cast of Shawn Christensen’s drama film The Vanishing of Sidney Hall, which premiered at the 2017 Sundance Film Festival and was released in 2018.

In February 2017, he was announced to have joined the cast of Bart Layton’s crime drama film American Animals. It premiered at the 2018 Sundance Film Festival on January 19, 2018 and was released on June 1, 2018, by The Orchard and MoviePass Ventures.

On June 27, 2018, it was announced that Jenner will be playing Christian de Neuvillette in the Goodspeed Musical production’s Cyrano at the Norma Terris Theatre from August 3 through September 9, 2018.

In August 2018, it was announced that Jenner had been cast as Sean in What/If, Netflix’s anthology neo-noir thriller series.

On August 29, 2019, it was announced that Jenner would star in Richard Linklater’s film adaptation of Merrily We Roll Along, which would be shot over the course of twenty years. Principal photography of the first section of the film has already been completed. Jenner is set to star alongside Beanie Feldstein and Ben Platt.

In 2019, Benoist strongly alluded to his domestic violence of her on her Instagram TV. Benoist suffered an injury in 2015 that tore her iris due to a phone being thrown at her eye by her then partner. They were confirmed married at the time. Benoist also stated she had experienced repeated instances of violence including being slapped, punched, shoved into a wall, dragged by her hair, and choked during the relationship. On October 8, 2020, Jenner responded, confirming the phone incident during a domestic dispute. He also alleged that the emotional, mental, and physical abuse during their relationship had been mutual stating that he had experienced being slapped, scratched, and a broken nose. Benoist had previously stated in her original Instagram video that she began fighting back against Jenner’s attacks during their relationship.