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In 2006, after featuring in minor credits in Motivos personales and Obsesión, Casas landed his first television main role in the teen drama SMS: Sin Miedo a Soñar and also debuted on the big screen in Summer Rain. Casas acquired a larger clout owing to his appearance in the television series Los hombres de Paco, as part of a popular love triangle together with Hugo Silva and Michelle Jenner. His early film career, featuring in box-office darlings such as Sex, Party and Lies (2009), Brain Drain (2009), and Three Steps Above Heaven (2010), consolidated after his performance as bad boy Hache in the latter title, which earned him wide popularity in Spain. He also starred in the television series El barco (2011−13).

He went on to feature in starring roles in films such as Unit 7 (2012), I Want You (2012; the sequel to Three Steps Above Heaven), The Mule (2013), Witching & Bitching (2013), Ismael (2013), My Big Night (2015), Palm Trees in the Snow (2015), Toro (2016), The Invisible Guest (2016), The Bar (2017), The Skin of the Wolf (2017), The Photographer of Mauthausen (2018), Bye (2019), The Occupant (2020), and The Paramedic (2020).

He also appeared in English-language international projects Eden and The 33.

Casas' leading performance in Cross the Line (2020) earned him his first Goya Award for Best Actor. He made his debut as a director with Caged Wings (2023). He received renewed industry recognition for his leading role in Away (2025).