Timothy Francis Leary was born in Springfield, Massachusetts, on October 22, 1920, and grew up as an only child in an Irish Catholic household. His father Timothy was a U.S. Army captain and his mother Abigail was a teacher.

Leary attended a number of educational institutions, including Holy Cross College (1938-39), the U.S. Military Academy (1940-41), and the University of Alabama where he earned his B.A. in 1943 while serving in the Army. He received his M.S. degree from Washington State University in 1946. Leary continued his intellectual pursuits at the University of California at Berkeley where, in 1950, he received a Ph.D. in clinical psychology.

After graduating from Berkeley, Leary stayed on as an assistant professor from 1950-55. He left this position to become director of clinical research and psychology at the Kaiser Foundation Hospital in Oakland, California, where he stayed until 1958. In 1959, Leary was appointed as a lecturer at Harvard University. During this period Leary introduced psilocybin to a number of the Beats, including Allen Ginsberg and Jack Kerouac, Peter Orlovsky, and Williams S. Burroughs. He also administered psilocybin to colleagues, students, and inmate volunteers finding that it was useful in the treatment of alcoholism, schizophrenia, and other psycho-physiological disorders. By 1963 Harvard, faced with controversy as a result of Leary's activities, dismissed him along with his colleague Richard Alpert.

After their dismissal from Harvard, Leary and Alpert founded a privately-financed research group in Mexico, called the International Foundation for Internal Freedom, to study and promote the use of LSD. However, the Mexican government soon closed them down and in August 1963, Leary moved his operation to a donated four-thousand-acre estate with a sixty-four-room mansion in Millbrook, near Poughkeepsie, New York.

From as early as 1962 until 1970, Leary had been arrested and incarcerated on drug-related charges in Mexico, British West Indies, Texas, New York, Michigan, and California. In April 1966, the Millbrook estate was raided by local police, led by G. Gordon Liddy then of the Dutchess County Sheriff's Department, and four people, including Leary, were arrested for possession of drugs. Following his arrest, Leary, to avoid constant harassment, founded the League for Spiritual Discovery which was a religious movement that sought constitutional protection for the right to take LSD as a sacramental substance.

In 1970, after being sentenced to ten years imprisonment in California to be served consecutively, not concurrently, with a Texas sentence, Leary was immediately sent to a minimum security prison near San Luis Obispo, California. However, by mid-September, Leary's third wife Rosemary, in conjunction with the radical Weathermen group, arranged for Leary's escape from prison. He was spirited to Algeria with his wife by the Weathermen, where they were granted political asylum; he details this experience in his book (1970). He and Rosemary took up residence in Algiers with fugitive Eldrige Cleaver and Cleaver's exiled Black Panther Party. By February 1971, a rift had developed between Cleaver and Leary, supposedly engineered by the FBI, so Leary left Algeria for Switzerland where he spent eighteen months before eventually arriving in Afghanistan.

In early 1973, Leary was kidnapped at gun point in Afghanistan by American agents. They brought him to California where he was found guilty of prison escape. He spent three more years in twenty nine jails in California's prison system. He was released on April 21, 1976, by Governor Jerry Brown.

Once Leary was released from prison in 1976, he spent most of his time at his home in Beverly Hills and on the campus lecture circuit where he took on a new role as a promoter of space colonization and life extension through scientific research to retard the aging process. Leary's activities during the late seventies included the formation of a cooperative to colonize space called Starseed and in 1982 he toured on a debate circuit with his former nemesis G. Gordon Liddy.

In 1995 Leary learned that he had inoperable cancer. He died amongst friends on May 31, 1996 at his home in Beverly Hills. On April 22, 1997, Leary's ashes were launched into space along with the ashes of 23 others, from Grand Canary Island off the Moroccan coast.