Marienthal was born in Santa Monica, California, the son of Penny Marienthal and Joseph Cross. He has two siblings, actors Harley Cross and Flora Cross. In addition to acting, Marienthal has written and performed his own work as a poet, dancer, and playwright.
Marienthal graduated from the private East Bay French-American School in Berkeley, where all students learn to speak fluent French and attend classes in two languages, and from Berkeley High School (Berkeley, California) in 2004. He graduated from Brown University Magna Cum Laude in 2008 with a double major in Comparative Literature and International Development Studies. He also holds a Masters degree in Development Studies from Brown. He is currently a doctoral candidate in the Geography Department at the University of California, Berkeley.
Marienthal's career started in Bay Area stage theater work, performing in Missing Persons, The Cryptogram, Hecuba, A Midsummer Night's Dream, Every 17 Minutes the Crowd Goes Crazy, and The Life of Galileo.