Early life Leary was born in Springfield, Massachusetts the son of an Irish American dentist, who abandoned the family when Timothy was a teenager. Leary studied briefly at the College of the Holy Cross in Worcester, Massachusetts, but reacted badly to the strict training at the Jesuit institution. He also attended West Point but was forced to resign after an incident involving smuggling liquor during a school field exercise and an extended period of a schoolwide "silent treatment." There is evidence that as one of the few Irish Catholics then attending West Point, he was made a scapegoat as his Protestant co-conspirators were allowed to continue their studies.

He earned a bachelor's degree in psychology at the University of Alabama in 1943, a master's degree at Washington State University in 1946, and a Ph.D. in psychology at the University of California, Berkeley in 1950. He went on to become an assistant professor at Berkeley (1950-1955), a director of research at the Kaiser Foundation (1955-1958), and a lecturer in psychology at Harvard University (1959-1963). Leary later described these years disparagingly, writing that he had been

an anonymous institutional employee who drove to work each morning in a long line of commuter cars and drove home each night and drank martinis .... like several million middle-class, liberal, intellectual robots.

Exploration of psychedelics On May 13, 1957, Life Magazine published an article by R. Gordon Wasson that documented (and popularized) the use of entheogens in the religious ceremony of the indigenous Mazatec people of Mexico. Influenced by Wasson's article, Leary traveled to Mexico, where he tried psilocybin mushrooms, an experience that drastically altered the course of his life. Upon his return to Harvard in 1960, Leary and his associates, notably Richard Alpert (later known as Ram Dass), began the Harvard Psilocybin Project conducting research into the effects of psilocybin and later LSD with graduate students.

Leary argued that LSD, used with the right dosage, set and setting, and preferably with the guidance of professionals, could alter behavior in unprecedented and beneficial ways. His experiments produced no murders, suicides, psychoses, and supposedly no bad trips. The goals of Leary's research included finding better ways to treat alcoholism and to reform convicted criminals. Many of Leary's research participants reported profound mystical and spiritual experiences, which they claim permanently altered their lives in a very positive manner.

Leary and Alpert were dismissed from Harvard in 1962. Their colleagues were uneasy about the nature of their research, and some parents complained to the university administration about the distribution of hallucinogens to their children. They founded the International Foundation for Internal Freedom in 1962 in Cambridge, Massachusetts. In 1963 they relocated to a large mansion in Poughkeepsie, New York called Millbrook and continued their experiments. Leary later wrote,

We saw ourselves as anthropologists from the twenty-first century inhabiting a time module set somewhere in the dark ages of the 1960s. On this space colony we were attempting to create a new paganism and a new dedication to life as art.

Repeated FBI raids ended the Millbrook era.

In 1964, Leary co-authored a book with Ralph Metzner called The Psychedelic Experience, ostensibly based upon the Tibetan Book of the Dead. In it he writes:

A psychedelic experience is a journey to new realms of consciousness. The scope and content of the experience is limitless, but its characteristic features are the transcendence of verbal concepts, of space-time dimensions, and of the ego or identity. Such experiences of enlarged consciousness can occur in a variety of ways: sensory deprivation, yoga exercises, disciplined meditation, religious or aesthetic ecstasies, or spontaneously. Most recently they have become available to anyone through the ingestion of psychedelic drugs such as LSD, psilocybin, mescaline, DMT, etc. Of course, the drug does not produce the transcendent experience. It merely acts as a chemical key - it opens the mind, frees the nervous system of its ordinary patterns and structures.

In 1966 he formed the League of Spiritual Discovery, a religion with LSD as their sacrament. This later became The Brotherhood of Eternal Love.

Leary later went on to propose his eight circuit model of consciousness, in which he claimed that the human mind consisted of eight circuits of consciousness. He believed that most people only access four of these circuits in their lifetimes. The other four, Leary claimed, were evolutionary off-shoots of the first four and were equipped to encompass life in space, as well as expansion of consciousness that would be necessary to make further scientific and social progress. Leary suggested that some people may shift to the latter four gears by delving into meditation and other spiritual endeavors. An example of the information Leary cited as evidence for the purpose of the "higher" four circuits was the feeling of floating and uninhibited motion experienced by users of marijuana. In the eight-circuit model of consciousness, a primary theoretical function of the fifth circuit (the first of the four developed for life in outer space) is to allow humans to become accustomed to life in a zero or low gravity environment.

Trouble with the law Leary's first run in with the law came in 1965. During a border crossing from Mexico into the United States, his daughter was caught with marijuana. After taking responsibility for the controlled substance, Leary was convicted of possession under the Marijuana Tax Act and sentenced to 30 years in jail. Soon after, however, he appealed the case, claiming the Marijuana Tax Act was in fact unconstitutional, as it required a degree of self-incrimination. Leary claimed this was in stark violation of the Fifth Amendment. The Supreme Court concurred. In 1969, the Marijuana Tax Act was declared unconstitutional, and Timothy Leary's conviction was quashed.

In 1970, Leary was again convicted of possession of marijuana and was sentenced to jail. When Leary arrived in prison, he was given psychological tests that were used to assign inmates to appropriate work details. Having designed many of the tests himself, Leary answered them in such a way that he seemed to be a very conforming, conventional person with a great interest in forestry and gardening.

As a result, Leary was assigned to work as a gardener in a lower security prison, which made escape possible. Leary considered his non-violent escape to be a humorous prank and left a challenging note for the authorities to find after he was gone. For a fee paid by The Brotherhood of Eternal Love, the Weathermen smuggled Leary and his wife Rosemary Woodruff Leary out of the United States and into Algeria. The couple's plan to take refuge with the Black Panther Eldridge Cleaver failed after Cleaver attempted to hold Leary hostage. Leary described his expectation of reasonableness from a black militant as "naive." The couple fled to Switzerland.

In 1974, having separated from Rosemary, Timothy Leary was "unlawfully captured" by U. S. government agents (some would say "kidnapped" as the United States had no extradition treaty with Afghanistan at the time and U. S. agents had no legal authority on Afghani soil) at an airport in Kabul, Afghanistan, and then flown to the United States. At a layover in the United Kingdom he requested political asylum from Her Majesty's Government, to no avail. He was then held on five million dollars bail ($21 mil. in 2006), the highest in U. S. history to that point; President Richard Nixon had earlier labeled him "the most dangerous man in America." He was incarcerated in Folsom Prison, in California, where at one point he was in a cell immediately adjacent to Charles Manson. He cooperated with the FBI's investigation of the Weathermen, becoming an informant who implicated friends and helpers in exchange for a reduced sentence. However, no one was ever prosecuted based on any information Leary gave to the FBI, as noted in an Open Letter from the Friends of Timothy Leary:

The Weather Underground, the radical left organization responsible for his escape, was not impacted by his testimony. Histories written about the Weather Underground usually mention the Leary chapter in terms of the escape for which they proudly took credit. Leary sent information to the Weather Underground through a sympathetic prisoner that he was considering making a deal with the FBI and waited for their approval. The return message was "we understand."

Leary appears to have been smart and audacious enough to have played along without compromising those who had helped him. This sort of escapade is in line with others throughout his life, such as his manipulation of psychological test responses that enabled him to get into a prison from which he could engineer his escape, and his confrontation of FBI agents who were terrifying an innocent young Hispanic woman during the Millbrook bust (led by G. Gordon Liddy), which was described in an eye-witness interview in the "Timothy Leary's Dead" (TLD) movie DVD (see below). Leary was released on April 21, 1976, by Governor Jerry Brown. At that time, Leary had spent more time for Cannabis possession than anyone else in USA history.

Further evidence of Leary's savvy was his cultivation of a friendship with former foe G. Gordon Liddy (whose former boss, Richard Nixon, had ordered him to destroy Leary), after his release from prison. At the time, both men were near financial insolvency, and Leary correctly guessed that they could earn money touring the country as ex-cons debating the soul of America.

Death In the months before his death from inoperable prostate cancer, Leary authored a book called Design for Dying, which attempted to show people a new perspective of death and dying.

For a number of years, Leary was excited by the possibility of freezing his body in cryonic suspension. As a scientist himself, he didn't believe that he would be resurrected in the future, but he recognized the importance of cryonic possibilities and was generally an advocate of future sciences. He called it his "duty as a futurist," and helped publicize the process. Leary had relationships with two cryonic organizations, the original ALCOR and then the offshoot CRYOCARE. When these relationships soured due to a great lack of trust, Leary requested that his body be cremated, which it was, and distributed among his friends and family.

Leary's death was videotaped for posterity, capturing his final words forever. This video has never been publicly seen but will be included in a documentary currently in production. At one point in his final delirium, he said, "Why not?" to his son Zachary. He uttered the phrase repeatedly, in different intonations and died soon after. His last word, according to Zach Leary, was "beautiful." With the movie Timothy Leary's Dead, filmmakers capitalised on his initial desire for cryogenic preservation by secretly creating a fake decapitation sequence without permission from Leary or his family, or so some claim. After the movie's release, the filmmakers declined to admit the scene's falsehood, possibly as a method to generate hype and sell tickets.

Seven grams of Leary's ashes were arranged by his friend at Celestis to be buried in space aboard a rocket carrying the remains of 24 other people including Gene Roddenberry (creator of Star Trek), Gerard O'Neill (space physicist), Krafft Ehricke (rocket scientist), and others. A Pegasus rocket containing their remains was launched on February 9, 1997.