He made his acting debut as Aquaman / Arthur Curry on The CW superhero series Smallville (2005–2010), where he appeared as a guest star between the fifth and tenth seasons.

He gained wider recognition for portraying the title character in the ongoing Amazon Prime Video action thriller series Reacher since 2022.

Outside television, Ritchson played Raphael in the 2014 Teenage Mutant Ninja Turtles reboot and its 2016 sequel, along with appearances in The Hunger Games: Catching Fire (2013), Lazer Team (2015), and Fast X (2023).

[8] He was granted a full music scholarship for singing, and started to attend college but dropped out "because [he] didn't feel fulfilled".

[14] Ritchson first gained celebrity status in 2004 when he appeared on American Idol as one of the top 87 contestants in the third season before being cut in Hollywood.

[19] In 2010 he started playing the main role of Thad Castle, the captain of a college football team, in Spike TV's Blue Mountain State.

In 2007, director Robert Zemeckis used Ritchson for his facial image, physique and movement for actor Ray Winstone of the title character of Beowulf.

In 2018 Ritchson was announced as the director, co-writer, and producer of Dark Web: Cicada 3301, a comedy-thriller film inspired by the eponymous organization.

[37] In 2024, Ritchson appeared in Jon Gunn's drama movie Ordinary Angels, written by Meg Tilly and Kelly Fremon Craig,[38] and in Guy Ritchie's action comedy film The Ministry of Ungentlemanly Warfare.

[39] Other projects include an independent album called This Is Next Time, released in late 2005, and appearing in a Russian commercial for Orbit gum.

[citation needed] Ritchson also appears in a web series titled Enormous Friends with his Blue Mountain State co-star, Rob Ramsay.

[45] On April 3, 2024, The Hollywood Reporter published an interview of Ritchson in which he says that he had once been sexually harassed by a "very famous photographer" and for this reason he left his modeling career.