Lebowitz was born and raised in Morristown, New Jersey, the daughter of Ruth and Harold Lebowitz, parents who owned a furniture store and upholstery workshop. She developed an obsessive love of reading from an early age, to the point that she would surreptitiously read during class and neglect her homework. Lebowitz describes her "Jewish identity [as] ethnic or cultural or whatever people call it now. But it's not religious. " She has been an atheist since the age of 7.