Kathy Boudin was born on May 19, 1943, in Manhattan, New York City. She grew up in a Jewish family with a strong background in left-wing politics. She was raised in Greenwich Village. Her paternal grandparents had come to America from Russia and Austria. Kathy’s family included many notable figures. Her great-uncle was Louis B. Boudin, a Marxist theorist. Her brother, Michael Boudin, became a conservative judge in the U.S.

Her mother, Jean Roisman Boudin, was a poet. Kathy's uncle by marriage was I.F. Stone, a well-known radical journalist. Her father, Leonard Boudin, was a civil rights lawyer. He represented many controversial clients, including Judith Coplon and the Cuban government. Leonard Boudin was a member of the National Lawyers Guild and worked alongside Victor Rabinowitz, who also supported left-leaning organizations.

Kathy was a bright student. She graduated from Bryn Mawr College in 1965 and was the valedictorian of her class. After college, she attended Case Western Reserve University School of Law for a short time, leaving before completing her studies.

In the 1970s, Kathy met David Gilbert, who became her partner. In 1980, they had a son named Chesa Boudin. When Chesa was just 14 months old, Kathy was arrested. She was later convicted and sent to prison for her involvement in a robbery. During her time in prison, Chesa was raised by friends of Kathy who were also involved in radical politics.

In 1981, Kathy Boudin took part in a robbery of a Brink's armored car in Nanuet, New York. This robbery involved former members of the Weather Underground, a group she was part of, along with members of the May 19th Communist Organization and the Black Liberation Army. During the heist, Boudin acted as a decoy and was in the front seat of a U-Haul truck used to help the thieves escape.

When police arrived and noticed the truck, they pulled it over. Boudin tried to appear innocent, urging the officers to lower their weapons. At that moment, her accomplices jumped out from the back of the truck with automatic weapons and opened fire. They killed two police officers, Edward O'Grady and Waverly Brown. Additionally, they seriously wounded a security guard, Joseph Trombino, and killed another guard, Peter Paige.

After the robbery, Boudin attempted to flee but was quickly arrested. She faced serious charges due to the deaths caused during the robbery. To avoid being convicted of triple murder, Boudin accepted a plea deal. She pleaded guilty to felony murder and robbery, which resulted in a sentence of 20 years to life in prison.