He starred as Ulises Garmendia in the series The Boat from 2011 to 2013. He has also starred in the films Sex, Party and Lies, Brain Drain, Neon Flesh, Three Steps Above Heaven, Unit 7, I Want You, The Mule, Witching and Bitching, and Ismael. In 2011 he won a Fotogramas de Plata Award for Most Searched Performer on the Internet. The same year he won Best New Actor from the Premio ACE Awards. In 2013 Casas won Best Actor at the Malaga Spanish Film Festival. He won another Fotogramas de Plata Award in 2013 for Best Movie Actor and took home a Feroz Award in 2014 for Best Actor in a Supporting Role. Mario has been in a relationship with actress Maria Valverde since 2009.