Mark O'Brien (born May 7, 1984) is a Canadian actor and director best known for his roles as Des Courtney in Republic of Doyle (2010–2014) and Tom Rendon in Halt and Catch Fire (2015–2016).

[4] At the 10th Canadian Screen Awards in 2022, he received two nominations for his film The Righteous, for both Best Supporting Actor and Best Original Screenplay.

[6] He is an English major with a Bachelor of Arts from Memorial University of Newfoundland.

O'Brien married actress Georgina Reilly on January 6, 2013, after having met on the set of Republic of Doyle in 2011.

[7] He is friends with fellow Newfoundlander Jonny Harris, who was also guest at his wedding.