Loken was born in Ghent, New York to Merlin "Chris" Loken (a writer) and Rande Porath (a model); both her paternal and maternal grandparents were born in Norway and immigrated to Wisconsin's Norwegian American community. She grew up on her parents' fruit farm in Upstate New York. She is also one of the tallest known actresses in the film industry today, standing at just under six feet tall (as stated in an interview from the Terminator 3 DVD's special features).

Loken started her acting career in 1996 on an episode of Star Trek: Voyager, and she appeared in several television shows and films, including regular appearances on the television shows Philly, Unhappily Ever After, Mortal Kombat: Conquest, and Boy Meets World. She is probably best known for her performance as the gynoid T-X (Terminatrix) in the 2003 movie Terminator 3: Rise of the Machines. In 2004, she appeared in a German television movie, Die Nibelungen (also known as "Sword of Xanten"), which aired as a 2-part mini-series and set a ratings record. Subsequently, she starred as the leading character in the 2006 film version of the video game BloodRayne, and appeared in director Uwe Boll's film version of the video game Dungeon Siege, called In the Name of the King. She will appear in 10 episodes of the fourth season of The L Word, which debuts in January 2007.