The Marin County coroner's office said Williams was last been seen alive at home at about 10 p.m. Sunday. An emergency call from his house in Tiburon was placed to the Sheriff's Department shortly before noon Monday.
Williams had been battling severe depression recently, said Mara Buxbaum, his press representative. Just last month, he announced he was returning to a 12-step treatment program he said he needed after 18 months of nonstop work. He had sought treatment in 2006 after a relapse following 20 years of sobriety.
U.S. President Barack Obama called Robin Williams "one of a kind" who shared his talent "freely and generously with those who needed it most."
From his breakthrough in the late 1970s as the alien in the hit TV show Mork and Mindy, through his stand-up act and such films as Good Morning, Vietnam, the short, barrel-chested Williams ranted and shouted as if just sprung from solitary confinement. Loud, fast, manic, he parodied everyone from John Wayne to Keith Richards, impersonating a Russian immigrant as easily as a pack of Nazi attack dogs.
He drew laughs in drag in Mrs. Doubtfire, or as a cartoon genie in Aladdin. He won his Academy Award in a rare, but equally intense dramatic role, as a teacher in the 1997 film Good Will Hunting.
He was no less on fire in interviews. During a 1989 chat with The Associated Press, he could barely stay seated in his hotel room, or even mention the film he was supposed to promote, as he free-associated about comedy and the cosmos.
"There's an Ice Age coming," he said. "But the good news is there'll be daiquiris for everyone and the Ice Capades will be everywhere. The lobster will keep for at least 100 years, that's the good news. The Swanson dinners will last a whole millennium. The bad news is the house will basically be in Arkansas."
Following Williams on stage, Billy Crystal once observed, was like trying to top the Civil War. In a 1993 interview with the AP, Williams recalled an appearance early in his career on The Tonight Show Starring Johnny Carson. Bob Hope was also there.
"It was interesting," Williams said. "He was supposed to go on before me and I was supposed to follow him, and I had to go on before him because he was late. I don't think that made him happy. I don't think he was angry, but I don't think he was pleased.
"I had been on the road and I came out, you know, gassed, and I killed and had a great time. Hope comes out and Johnny leans over and says, 'Robin Williams, isn't he funny?' Hope says, 'Yeah, he's wild. But you know, Johnny, it's great to be back here with you."'
In 1992, Carson chose Williams and Bette Midler as his final guests.
Like so many funnymen, he had serious ambitions, winning his Oscar for his portrayal of an empathetic therapist in Good Will Hunting. He also played for tears in Awakenings, Dead Poets Society and What Dreams May Come, something that led New York Times critic Stephen Holden to once say he dreaded seeing the actor's "Humpty Dumpty grin and crinkly moist eyes."
Williams also won three Golden Globes, for Good Morning, Vietnam, Mrs. Doubtfire and The Fisher King.
His other film credits included Robert Altman's Popeye (a box office bomb), Paul Mazursky's Moscow on the Hudson, Steven Spielberg's Hook and Woody Allen's Deconstructing Harry. On stage, Williams joined fellow comedian Steve Martin in a 1988 Broadway revival of Waiting for Godot.
"Robin was a lightning storm of comic genius and our laughter was the thunder that sustained him. He was a pal and I can't believe he's gone," Spielberg said.
More recently, he appeared in the Night at the Museum movies, playing President Theodore Roosevelt in the comedies in which Ben Stiller's security guard has to contend with wax figures that come alive and wreak havoc after a museum closes. The third film in the series is in post-production, according to the Internet Movie Database.
In April, Fox 2000 said it was developing a sequel to Mrs. Doubtfire and Williams was in talks to join the production.
Williams also made a short-lived return to TV last fall in CBS's The Crazy Ones, a sitcom about a father-daughter ad agency team that co-starred Sarah Michelle Gellar. It was cancelled after one season.
"I dread the word `art,"' Williams told the AP in 1989. "That's what we used to do every night before we'd go on with 'Waiting for Godot.' We'd go, 'No art. Art dies tonight.' We'd try to give it a life, instead of making Godot so serious. It's cosmic vaudeville staged by the Marquis de Sade."
His personal life was often short on laughter. He had acknowledged drug and alcohol problems in the 1970s and '80s and was among the last to see John Belushi before the Saturday Night Live star died of a drug overdose in 1982.
Williams announced in recent years that he was again drinking but rebounded well enough to joke about it during his recent tour. "I went to rehab in wine country," he said, "to keep my options open."
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 and he was accepted into the Juilliard Academy, where he had several classes in which he and Christopher Reeve were the only students and John Houseman was the teacher.
Encouraged by Houseman to pursue comedy, Williams identified with the wildest and angriest of performers: Jonathan Winters, Lenny Bruce, Richard Pryor, George Carlin. Their acts were not warm and lovable. They were just being themselves.
"You look at the world and see how scary it can be sometimes and still try to deal with the fear," he told the AP in 1989. "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."
After appearing as a supporting player on The Richard Pryor Show, he unveiled Mork, the alien from the planet Ork, in an appearance on Happy Days, and was granted his own series, co-starring Pam Dawber, from 1978-82.
"I am completely and totally devastated," Dawber said in a statement. "What more can be said?"
In subsequent years, Williams often returned to television — for appearances on Saturday Night Live, for Friends, for comedy specials, for American Idol, where in 2008 he pretended to be a "Russian idol" who belts out a tuneless, indecipherable My Way.
Williams also could handle a script, when he felt like it, and also think on his feet. He ad-libbed in many of his films and was just as quick in person. During a media tour for Awakenings, when director Penny Marshall mistakenly described the film as being set in a "menstrual hospital," instead of "mental hospital," Williams quickly stepped in and joked, "It's a period piece."
Along with Crystal and Whoopi Goldberg, Williams helped found the American edition of Comic Relief, a series of concerts and television specials that helped raise money to address homelessness.
Winner of a Grammy in 2003 for best spoken comedy album, Robin Williams — Live 2002, he 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.
"You have an internal critic, an internal drive that says, `OK, you can do more.' Maybe that's what keeps you going," Williams said. "Maybe that's a demon. ... Some people say, `It's a muse.' No, it's not a muse! It's a demon! HAHAHAHAHAHAHA!!! THE LITTLE DEMON!!"