Harrelson was born in Midland, Texas to Charles Voyde Harrelson and Diane Lou Oswald, who divorced in 1964; he has two brothers, Jordan and Brett, who is a professional motorcycle racer. His father, a professional hitman, was twice convicted for murder for hire, and was jailed for most of Harrelson's childhood for performing a hired killing[1]. Harrelson grew up in Lebanon, Ohio with his deeply religious mother, and has frequently said that his father's past has colored his own present.

Harrelson attended Lebanon High School and later Hanover College in Indiana, where he studied drama and received a Bachelor of Arts in Theater Arts and English in 1983.

After graduation, Harrelson moved to New York City, and became an understudy in Neil Simon's Biloxi Blues. In 1985, he was cast as the naive but genial Midwestern bartender Woody Boyd on the television series Cheers, and won the "Funniest Newcomer" American Comedy Award and an Emmy for the role. His first film was 1986's Wildcats with Goldie Hawn. He appeared in mostly minor roles until he played Michael J. Fox's romantic rival in 1991's Doc Hollywood. Subsequently, Harrelson has appeared in such films as Money Train, White Men Can't Jump, The Cowboy Way, Indecent Proposal, The Hi-Lo Country, Natural Born Killers and Kingpin. In 1996, he starred in the title role of the controversial film The People vs. Larry Flynt, for which he received a nomination for the Academy Award for Best Actor. More recently, he had a fairly long run on the NBC sitcom Will and Grace as Grace's love interest Nathan. He also played a transvestite prostitute in Anger Management, and FBI agent Stan in 2004's After the Sunset. His most recent film is A Prairie Home Companion, which was released in June of 2006.

In 1985, Harrelson married Nancy Simon, daughter of playwright Neil Simon, in Tijuana. The two intended to divorce the following day, but the storefront marriage/divorce parlor was closed when they had returned to it, and the two remained married for ten months.[2] On January 11, 1998, Harrelson married Laura Louie, his former assistant and a co-founder of Yoganics, an organic food delivery service.[3] The couple, who have been together since 1990, have three daughters, Deni Montana (born March 5, 1993), Zoe Giordano (born September 22, 1996), and Makani Ravello (born June 3, 2006). When announcing Makani's birth, the couple referred to the three as their "goddess trilogy."[4]

Harrelson is an outspoken supporter for the legalization of marijuana and hemp in the USA. [5] On June 1, 1996, he was intentionally arrested in Kentucky after symbolically planting four hemp seeds to challenge state law that failed to distinguish between industrial hemp and marijuana; he won the case. An environmental activist, he once climbed the Golden Gate Bridge in San Francisco to unfurl a banner that said, "Hurwitz. Aren't ancient redwoods more precious than gold?" in protest of MAXXAM/Pacific Lumber CEO Charles Hurwitz, who once stated, "He who has the gold, rules". Harrelson, an ethical vegan and raw foodist, has also denounced animal experiments in the cosmetics industry. He has travelled the American West Coast on a bike in caravan with a hemp oil-fueled biodiesel bus (the subject of the independent documentary, Go Further) and has narrated the documentary Grass (1999). He is also an antiwar activist and has often spoken publicly against the 2003 invasion of Iraq.