In 2007, Stevenson became an actor and took his first film role in Fred Claus as Young Nick. He then appeared in the black comedy In Bruges in 2008. In 2010 he starred in Reuniting the Rubins where he played the role of Jake Rubins. In 2011 he took on his best-known role as Horrid Henry in Horrid Henry: The Movie where he worked alongside Tyger Drew-Honey and Anjelica Huston. He also appeared in the film's music video and appeared on Daybreak in late 2011.
A year after Horrid Henry: The Movie, Vertigo Films announced that Stevenson would star in an upcoming secondary school version of Streetdance called Streetdance Juniors which was renamed All Stars in June 2012, releasing on 3 May 2013. He stars alongside Akai Osei, Ashley Jensen, Kimberley Walsh and John Barrowman amongst others.
From 2014 to present, Stevenson has a main role in Millie Inbetween, a CBBC comedy-drama, playing the role of the main character Millie's stepbrother.
From 2015 to present, Stevenson has a main role as Toby Hawkins in the drama Humans where he has a crush on his family's synth, Anita. Theo has a twin sister called Madeleine Stevenson and her mother and father Sarah and John Stevenson.
|2007||Fred Claus||Young Nick||Debut Role|
|2008||In Bruges||Boy in Church||Film|
|2011||Horrid Henry: The Movie||Horrid Henry||Film|
|2012||Little Cracker||Boy||Darren Boyd's episode|
|2013||All Stars (2013 film)||Ethan||Film|
|2014||Millie Inbetween||Craig||TV series|