Knopf was born into a Jewish family in New York City. His father, Samuel Knopf, was an advertising executive and financial consultant, his mother was Ida Japhe, a school teacher. Samuel Knopf was originally from Warsaw, Poland, but came to New York with his parents, where he worked his way up the directorship at a small mercantile bank. Alfred's mother, Ida, was from a Latvian Jewish family who settled in New York. For a time Knopf's parents lived in the Midwest and in Virginia. Ida committed suicide when Alfred was five years old and his sister Sophia was almost two. That same day, Alfred's father had filed for divorce in which he named Ida as an adulteress. His father later married Lillian Harris, who had a daughter, Bertha, from a previous marriage. With Lillian, Samuel had another son, Edwin H. Knopf, who worked for Alfred briefly, then became a film director and producer.