Ellenberg was born in Potomac, Maryland. He was a child prodigy who taught himself to read at the age of two by watching Sesame Street. His mother discovered his ability one day while she was driving on the Capital Beltway when her toddler informed her: "The sign says 'Bethesda is to the right.'" In second grade, he helped his teenage babysitter with her math homework. By fourth grade, he was participating in high school competitions (such as the American Regions Mathematics League) as a member of the Montgomery County math team. And by eighth grade, he had started college-level work.
