O'Halloran's first role was as main character Dante Hicks in Kevin Smith's film Clerks, in 1994.

[1] He has returned many times as Dante or another of the Hicks family in films by Smith.

O'Halloran is the lead actor in the 2000 film Vulgar, about a small town clown who is traumatised after he is attacked during one of his performances.

[citation needed] In 2020, O'Halloran began presenting his own pop culture podcast, The O'HalloRant, on YouTube.

[3] A first-generation Irish-American, with parents from Galway and Roscommon, O'Halloran was born in Manhattan, New York City, and lived in Old Bridge Township, New Jersey from the age of 13.