She was born in Los Angeles. She was educated at Phillips Academy, Andover in Massachusetts. She received her BA in French and German comparative literature from Barnard College of Columbia University and her MFA in Acting from the Yale School of Drama. She currently resides in Brooklyn, New York.
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.
Marnò also directs and writes screenplays. Her first feature-length screenplay, When the Lights Went Out, was a quarter finalist for the Nicholl Fellowship, a finalist at the Nantucket Screenwriter's Colony, Sundance Screenwriter's Lab, and won third prize at the Cinequest Film Festival Screenwriting Competition. She has adapted When the Lights Went Out as a play and it received its first production at New York Stage & Film in July 2013 starring Laura Innes from ER.
Marnò's short film, Incoming, which she wrote and directed, was accepted to the Noor Iranian Film Festival, LA SHORTS FEST, the Asians on Film Festival, and the DC Asian Pacific American Film Festival.
She can be seen as reporter Ayla Sayyad on seasons two and three of the acclaimed Netflix series House of Cards, and as Mossad agent and assassin Samar Navabi, in seasons 2 and 3 of The Blacklist.
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.
She appeared in the pilot episode of Madam Secretary in 2014, but did not continue in that show due to her casting in The Blacklist.
In March 2019, and after nearly five seasons playing FBI Agent Samar Navabi, Marnò made the decision to leave The Blacklist. Because of this, her character was written out of the series and her final episode was The Osterman Umbrella Company (Season 6, Episode 14).