While still a Slamball player, Garrett landed a role on an episode of One Tree Hill that had a Slamball-related storyline. After that, he signed with an agent and continued to get minor roles in TV shows like Hawthorne, CSI: Miami, and NCIS, as well as films like Transformers: Dark of the Moon. In 2011, Garrett started a recurring role as Deputy Sheriff Cane on FX’s Sons of Anarchy. He appeared in 17 episodes from 2011 to 2014.

In 2016, Garrett landed two key roles; the first one, as Lieutenant TAO Cameron Burk on TNT’s The Last Ship, and the second as Secret Service agent Mike Ritter on ABC’s Designated Survivor. David Guggenheim, creator and executive producer of the latter, said they “needed someone who could throw President Tom Kirkman into a car”, but that had “an emotional core goodness”. The character of Mike Ritter became a fan favorite and Garrett remained with the show through the whole first two seasons, until its initial cancelation. After the show was picked up by Netflix, Garrett confirmed on Twitter that after “lengthy negotiations” he would not return due to budget constraints.

Garrett married Mina Ivanova on April 27, 2017. The couple had been dating for several years prior.

In late 2018, he was cast as Mar Novu / The Monitor in the Arrowverse crossover event “Elseworlds” on The CW, which connects storylines from The Flash, Arrow, and Supergirl. He reprised the role in 2019 on Arrow, The Flash, Supergirl, and Legends of Tomorrow. He was later upgraded to a series regular for those Arrowverse shows, as well as Batwoman, to ensure his availability for all appearances prior to and in the Crisis on Infinite Earths crossover. In July 2019, it was announced that Garrett would also be portraying the Anti-Monitor in the crossover.