His directorial effort was with the 2000 action film Highlander: Endgame. His other credits include Animals and The Day.
Aarniokoski has directed 8 installments of the CBS series Criminal Minds since 2011, while directing two episode of The CW's Green Arrow origin series Arrow, and one of its sister-series, The Flash.
He co-wrote and directed the 2013 horror Nurse 3D.
During the first season of the FOX series Sleepy Hollow, Aarniokoski directed the 7th episode, "The Midnight Ride". In winter 2014, he joined as supervising Producer and main Director on the show. He has since directed the second-season episodes "Go Where I Send Thee...", "And the Abyss Gazes Back", "Kali Yuga" and "Awakening".