Cronin was vocalist for the gothic rock band The Marionettes, which formed in 1986 and toured with bands such as Pearl Jam and Nirvana The band released four albums between 1990 and 1998. In 1999, Cronin began his acting career playing a High Priest in The Mummy. He subsequently had small roles in The World Is Not Enough and Harry Potter and the Chamber of Secrets. In 2015 he played a villain in Mission: Impossible - Rogue Nation. He also appeared as Max, one of the villainous Ferrino brothers (along with Bruce Payne as Ray), in The Antwerp Dolls. In 2016 he played the eponymous villain in Kill Kane, opposite Vinnie Jones. He has also been cast in Fantastic Beasts and Where to Find Them, based on J. K. Rowling's novel Fantastic Beasts and Where to Find Them.
Cronin's first directorial film was Escape to Gossau, which he co-directed with Michael Koltes. Cronin is currently directing The Dark Return of Time, based on the novel by R.B. Russell, starring Eric Roberts and Adrian Paul. Cronin is set to direct Give Them Wings, which focuses on the life of disabled writer Paul Hodgson, and Michael, a biopic of boxer Michael Watson.