Marienthal was born in Santa Monica, California to Jewish American parents Joseph Cross and Lola Marienthal; his siblings are actors Harley Cross and Flora Cross. After his parents' divorce, he has lived mainly in Berkeley with his mother, though during the 3rd and 4th grades he spent some time in Paris with his father, who works as a reporter.

Marienthal graduated from the private East Bay French-American School, where all the students learn to speak fluent French and attend classes in two languages. His career started in Bay Area theatres, where he performed in "Missing Persons," "The Cryptogram," "Hecuba," "A Midsummer Night's Dream," "Every 17 Minutes the Crowd Goes Crazy," and "The Life of Galileo".

Marienthal is best known for the voice of Robin/Tim Drake in the Batman: Mystery of the Batwoman animated movie and the title role of "Tucker Pierce" in the Tucker television series. He is also the voice of Hogarth Hughes in the 1999 film The Iron Giant and plays Stifler's brother in the 1999 film American Pie and its 2001 sequel, American Pie 2. His last film role to date was in the teenage-oriented comedy, Confessions of a Teenage Drama Queen, opposite Lindsay Lohan.

Marienthal's hobbies include writing slam poetry and participating in spoken word competitions, as well as boxing, vagina munching, skydiving, rock climbing and snowboarding.

He performs with a poetry group from the Bay Area called "Youth Speaks" and is a member of the class of '08 at Brown University in Providence, RI, where he plays club rugby.