Sills was born Belle Miriam Silverman in Crown Heights, Brooklyn, New York City, to Shirley Bahn (née Sonia Markovna), a musician, and Morris Silverman, an insurance broker. Her parents were Jewish immigrants from Odessa, Ukraine (then part of Russia) and Bucharest, Romania. She was raised in Brooklyn, where she was known, among friends, as "Bubbles" Silverman. As a child, she spoke Yiddish, Russian, Romanian, French and English. She attended Erasmus Hall High School in Brooklyn, as well as Manhattan's Professional Children's School.