The Marin County Sheriff's Office said he appears to have committed suicide, with asphyxiation a possible cause of death.
Authorities were called to Williams' home in Tiburon about noon Monday. The actor was found dead.
Williams, hailed as a comic genius, was a star of movies and television for more than three decades.
Williams, however, was open about his problems with cocaine and alcohol over the years.
The actor spent time on a Hazelden campus in Oregon in 2006. He later explained that drinking had gradually become a problem again after 20 years of sobriety.
This summer, he returned to rehab to "fine-tune" his sobriety.
The actor "has been battling severe depression of late," his publicist Mara Buxbaum said. "This is a tragic and sudden loss. The family respectfully asks for their privacy as they grieve during this very difficult time."
Neighbors were stunned by his death.
Kelly Cook, 50, who lives down the street, got a call from her mother while she was at work at a Sausalito educational company. She went to the store, chose orange gerbera daisies and was the first to drop a bouquet at the gate.
Williams and his family were Cook's neighbors before they moved two blocks down the way. She called him "brilliant," as well as "really quiet and private." An avid cyclist, he was often seen riding the Paradise loop, which hugs the Tiburon peninsula.
Cook's children called him "the funny man" and would greet him as such when he was out walking his pug, Lenny (after another famous comedian). He joked around easily with them, Cook said, "because they were kids."
Dubbed “the funniest man alive” by Entertainment Weekly in 1997, Williams brought audiences hours of laughter, putting his imaginative spin on characters in film and television. He was praised for his serious roles as well, winning a best supporting actor Oscar for his performance as Sean Maguire, the therapist who counsels Matt Damon’s math genius in “Good Will Hunting” (1997). He also received nominations for “The Fisher King” (1991), “Dead Poets Society” (1989) and “Good Morning, Vietnam” (1987).
"You're standing at a precipice and you look down, there's a voice and it's a little quiet voice that goes, 'Jump,'" the "Mrs. Doubtfire" star told ABC News in October of that year. "The same voice that goes, 'Just one.' … And the idea of just one for someone who has no tolerance for it, that's not the possibility.
Born in Chicago in 1951, Williams was accepted into John Houseman’s acting program at Juilliard along with Christopher Reeve, who became a lifelong friend.