Leonidas first appeared on television in 2001. He appeared in a single episode of Casualty portraying the character of Ross. In the same year shortly after appearing in Casualty, he got the role of Josh Irvine in the BBC series Grange Hill. He played the role of Josh for four years finishing in 2004. Leonidas also appeared in The Bill twice but as two different characters. He played the role of Josh Harvey in 2002 and then returned in 2005 to portray a completely new character called Ryan Greaves. In 2007 he appeared in Doctors as Mike Andreus in the episode "Chemistry", as well as Holby City that same year. In 2008 he played the role of Scott White in the TV series/documentary Banged Up Abroad.
In the 2009 horror/slasher Tormented he appeared alongside Alex Pettyfer, Georgia King and former Skins stars April Pearson and Larissa Wilson. His character, Alexis, is one of the most popular boys in school who, along with his sadistic friends, bullied a classmate to suicide.
He has a role in Neil Marshall's Centurion. In 2011 he appeared in the second half of series 6 of Doctor Who in the episode "The God Complex" as Howie Spragg. In 2014, Leonidas starred in the Second World War drama The Monuments Men, and starred in the biographical drama Rosewater.
