Pais was born in Amsterdam, the first child of middle-class Dutch Jewish parents. His father, Isaiah "Jacques" Pais, was the descendant of Sephardic Jews who migrated from Portugal to the Low Countries around the beginning of the 17th century. His mother, Kaatje "Cato" van Kleeff, was the daughter of an Ashkenazi Diamond cutter. His parents met while studying to become elementary-school teachers. They both taught school until his mother quit when they married on December 2, 1916. His only sibling, Annie, was born on November 1, 1920. During Pais's childhood his father was an elementary schoolmaster, headmaster, and later the headmaster of the Sephardic Hebrew school.