Marnò played the title role in the 2008 film The Stoning of Soraya M., about a woman whose husband falsely accuses her of adultery, resulting in her death by stoning. In addition she has had roles in a number of television series including The Paul Reiser Show, The Glades, Hung, The Mentalist, Bones, The Unit, Medium, K-ville, and Standoff. Marnò also appeared in the Untitled John Wells Medical Drama Pilot, which was not aired. She also stars in Ana Lily Amirpour's directorial debut, A Girl Walks Home Alone at Night, produced by Elijah Wood, under his company, The Woodshed. In 2011, Marnò voiced Mirabelle Ervine, a Breton mage and Master Wizard of the College of Winterhold, in the critically acclaimed video game The Elder Scrolls V: Skyrim.