Harriet Fleischl Pilpel was born in New York City in 1911 to businessperson Julius and Ethel (Loewy) Fleischl. She was graduated from Vassar in 1932 and received a Master's degree in 1933 and a law degree in 1936, both from Columbia University.

She married Robert Pilpel, a social service executive on June 15, 1933. They had two children, Judith Ethel Pilpel Appelaum and Robert Harry Pilpel.

Harriet Pilpel was a member of the New York Bar and senior partner in the law firm of Greenbaum, Wolff & Ernst who specialized in First Amendment issues. Between 1937 and 1991, she participated in twenty-seven cases before the U.S. Supreme Court. A prolific lecturer and author of publications related to copyright, abortion, marriage, birth control, civil liberties, and the law, she was also active in many organizations including the board of directors of Planned Parenthood Federation of America for whom she was legal counsel. Pilpel was also counsel to a number of other associations, including Sex Information and Educational Council of the United States, Association for the Study of Abortion, and Association for Voluntary Sterilization. In addition, she served on the boards of the National Abortion Rights Action League, the American Civil Liberties Union, and The Alan Guttmacher Institute. She died in her sleep April 23, 1991.