Marin County Sheriff's Office said it suspected the death was a suicide due to asphyxia, but the cause of death remains under investigation.
Williams's third wife Susan Schneider released a statement on Monday night in which she said: "This morning, I lost my husband and my best friend, while the world lost one of its most beloved artists and beautiful human beings. I am utterly heartbroken.”
The star’s publicist Mara Buxbaum said Williams had recently been suffering from severe depression.
In his final posting on Instagram on 1 August, Williams had wished his daughter Zelda a happy 25th birthday. Late on Monday, Zelda paid tribute to her father with an excerpt from French poet and aviator Antoine de Saint-Exupery's El Principito, which read: "You - you alone will have the stars as no one else has them... In one of the stars I shall be living.
"In one of them I shall be laughing. And so it will be as if all the stars were laughing, when you look at the sky at night ... You - only you - will have stars that can laugh."
She added: "I love you. I miss you. I'll try to keep looking up."
Williams, who had struggled with addictions to drink and drugs in the past, had entered a Minnesota rehabilitation centre last month to help him maintain sobriety.
His representatives at the time said Williams was not using drugs or alcohol but had gone to the centre to "fine-tune and focus" his sobriety after working a longer-than-usual schedule.
The Marin County Sheriff's office said it received an emergency call about midday on Monday, saying that Williams was unconscious and not breathing at his home near Tiburon, north of San Francisco. Local newspapers reported that he was pronounced dead shortly afterwards.
Fellow comedy star Steve Martin said in a tweet: "I could not be more stunned by the loss of Robin Williams, mensch, great talent, acting partner, genuine soul."
The thrice-married father-of-three was known to millions for his performances in films that ranged from the out and out manic comedy of Mrs Doubtfire, to more thought provoking fare like Dead Poets Society and The Fisher King. In many films, like Good Morning, Vietnam, Williams attracted widespread praise by successfully combining his gift for comedy with an exploration of more serious underlying themes.
Nominated for the Best Actor Oscar three times, Williams won the Academy Award for Best Supporting Actor for the 1997 film Good Will Hunting, in which he played the therapist to Matt Damon’s working class maths genius.
As a stand-up, his explosive, improvisational talent was so hard to match that even the established star Billy Crystal once admitted that following Williams on stage was like trying to top the Civil War.
Such was his talent that he was allowed to ad-lib in many of his films – and his quick wit enlivened many a media appearance.
During a media tour for the 1990 film Awakenings, when director Penny Marshall mistakenly described the film as being set in a "menstrual hospital," instead of "mental hospital," Williams had the presence of mind to offer the explanation: "It's a period piece."
Williams once likened his act to the daily jogs he took across the Golden Gate Bridge. There were times he would look over the edge, one side of him pulling back in fear, the other insisting he could fly.
He also said: "Comedy can deal with the fear and still not paralyze you or tell you that it's going away. You say, OK, you got certain choices here, you can laugh at them and then once you've laughed at them and you have expunged the demon, now you can deal with them. That's what I do when I do my act."
Born in Chicago in 1951, Williams would remember himself as a shy kid who got some early laughs from his mother - by mimicking his grandmother. He opened up more in high school when he joined the drama club.
He began his career in stand-up comedy before first achieving widespread fame in the TV sitcom Mork and Mindy. The show ran from 1978 to 1982 and Williams was frequently allowed to indulge his talent for improvised comedy while playing Mork, an alien living on Earth.
As Mork and Mindy became increasingly popular, Williams started to reach an even wider audience with a series of televised stand-up comedy shows.
During the late Seventies and early Eighties, however, Williams developed an addiction to cocaine.
Williams was a close friend of the comedian John Belushi who died of a cocaine and heroin overdose in 1982. Williams had enjoyed wild parties with Belushi and subsequently admitted that the death of his friend and the birth of his son prompted him to quit drugs: "Was it a wake-up call? Oh yeah, on a huge level.”
On August 9, 2006, however, Williams checked himself into a rehab centre, later admitting he was an alcoholic. His publicist announced: “After 20 years of sobriety, Robin Williams found himself drinking again and has decided to take proactive measures to deal with this for his own well-being and the well-being of his family.”
Williams, though, appeared to have recovered well enough to joke, on a recent tour: "I went to rehab in wine country. To keep my options open."
In March 2009 Williams was also hospitalised by heart problems, and had to undergo surgery to surgery to replace his aortic valve.
A keen charity fundraiser Williams had, with his second wife, Marsha, founded the Windfall Foundation, to raise money for a wide variety of causes.
The US President Barack Obama led an outpouring of tributes to Williams, calling the Hollywood Star a "one of a kind" who "arrived in our lives as an alien - but ended up touching every element of the human spirit".
The Hollywood actor Kevin Spacey tweeted: “Robin Williams made the world laugh & think. I will remember & honour that. A great man, artist and friend. I will miss him beyond measure.”
Steve Carell, the star of The 40-Year-Old Virgin and the US version of The Office, simply wrote: “Robin Williams made the world a little bit better. RIP.”
Williams will appear in upcoming film "Night at the Museum: Secret of the Tomb," playing the statue of Teddy Roosevelt who comes to life at night, and holiday comedy "Merry Friggin' Christmas." He was also attached to a sequel to 1993 hit "Mrs. Doubtfire."