Roy Cohn was born in Manhattan, New York. He graduated from Columbia Law School at the age of 20. Since his father was Judge Albert Cohn of the NY Supreme Court, getting his first job as an attorney was not hard to get. He began working for the office of US Attorney for Irving Saypol in Manhattan.

As Saypol's assistant at the U.S. Attorney's office in Manhattan, Cohn helped to win a number of high-profile anti-Communist cases. He was known for his eager prosecution of William Remington, who was a former Commerce Department employee that was convicted of perjury relating to his membership in the Communist Party. He also was responsible for the prosecution of eleven Communist Party leaders for attempting to overthrow and destroy the government of the United States by force and violence in speech or writing under the Smith Act. In addition, he is responsible for his work in the Alger Hiss case. Cohn prosecuted Alger Hiss, who was a U.S. State Department official and Secretary General to the founding charter conference of the United Nations, for perjury in 1950. But Cohn was most famous for his role in the 1951 espionage trial of Julius and Ethel Rosenberg. Cohn's cross examination of Ethel's brother produced the testimony that was mostly responsible for the Rosenbergs' conviction and execution. Interesting enough, that testimony later came to surface as false.

The Rosenberg trial brought the 24-year-old Cohn to the attention of FBI director J. Edgar Hoover, who recommended him to Joseph McCarthy. McCarthy was the chairman of the Government Committee on Operations of the Senate. His primary job was to investigate various government departments and question a large number of people about their political past. Some people lost their jobs after they admitted they had been members of the Communist Party. McCarthy made it clear to the witnesses that the only way of showing that they had abandoned their socialism views was by naming other members of the party. These investigations became known as McCarthyism.

Cohn soon became McCarthy's chief counsel. Surprisingly, he was chosen over Robert Kennedy. He gained power nearly equal to McCarthy's in the Senate Subcommittee on Investigations. He became known for his aggressive questioning of suspected Communists. He tended to hold the hearings in closed forums. This mixed well with McCarthy's preference for holding "executive sessions" and "off-the-record" sessions far away from the Capitol in order to minimize public inquiry. He was given free rein in pursuit of investigations. One tactic used by both Cohn and McCarthy was summoning gay men in the arts and threatening to expose them if they did not produce a list of suspected Communists. Government officials were targeted for homosexual tendencies and sometimes Cohn even used sexual secrets as a blackmail tool to gain informants.

In 1954, McCarthy began looking for suspects in the U.S. Army. The president at that time was Dwight D. Eisenhower. He was angry so he arranged to have the hearings televised so the American public could see McCarthy’s extreme bullying tactics. McCarthy arranged for Cohn's long-time friend, G. David Schine, to be given leave from the Army to serve on the subcommittee. That fueled clashes with the Army, which later contributed to McCarthy's public discrediting. As a result of the public shock and outrage, the Senate censured and discredited McCarthy in December 1954.

After McCarthy was censured in 1954, Cohn resigned and went into private practice. After leaving McCarthy, Cohn built a highly successful 30-year career as a high-powered attorney in New York City. His clients included Donald Trump, Mafia figures Tony Salerno and John Gotti, and the Archdiocese of New York. He maintained close ties with conservative politics, serving as an informal advisor to Presidents Richard Nixon and Ronald Reagan. He lost his law license in 1986 on grounds of unethical and unprofessional conduct.

When the NBC television network produced a movie in the early 1980s about McCarthy's career, Cohn responded by writing a paperback book criticizing the movie for factual errors and defending McCarthy's anti-Communist crusade.

Ironically, Cohn spent years targeting government officials and cultural figures for homosexual tendencies, but he was a closeted homosexual himself. He frequently went to gay bars, but always publicly denied being a gay. He encouraged rumors of a relationship with his long-time friend Barbara Walters, who publicly stated that she thought he was heterosexual. He died of HIV at the National Institute of Health Hospital in Bethesda, Maryland in 1986.