He first achieved notice for his stand-up routines in San Francisco. Garry Marshall cast him as the alien Mork in a guest star part in Happy Days that was so popular it led to his own television series, Mork and Mindy, which ran from 1978 to 1982. The majority of his acting career has been for cinema, although he has made some memorable performances on stage as well (notably as Estragon in a production of Waiting for Godot).

His first starring roles in Popeye and in The World According to Garp were both major flops, but with Good Morning Vietnam Williams established a screen identity. Many of his roles have been comic (e.g. The Birdcage, Mrs. Doubtfire). In particular, his role as the Genie in the animated film Aladdin was instrumental in establishing the importance of star power in voice actor casting. Later Williams once again used his voice actor talents in the 2005 animated feature Robots. He has also starred in serious movies (Awakenings , What Dreams May Come) and in 1998 he won an Oscar as Best Supporting Actor for his role as a psychologist in Good Will Hunting. However, by the early 2000s, his choices for dramatic roles grew to be criticized as overly mawkish and sentimental, especially 1998's Patch Adams, prompting him to temporarily dabble in villainous roles such as in Insomnia and One Hour Photo.

He is known for his wild improv skills and impersonations. He is a talented mimic and can jump in and out of characters at an extremely fast pace. His comic style is a major influence on late night talk show host/comedian Conan O'Brien. Williams states that he began doing impersonations as a child mimicking the Southern accent of his aunt.

Robin Williams and his wife Marsha Garces Williams founded the Windfall Foundation, a philanthropic organization to raise money for many different charities. Robin Williams devotes much of his energy doing work for charities, including the Comic Relief fund-raising efforts.

Williams first marriage was to Valerie Velardi on June 4, 1978, with whom he had one child, Zachary. Williams apparently fell in love with his son's nanny, Marsha Garces, which resulted in the end of his first marriage in 1988. He subsequently married for the second time, this time to Marsha Garces Williams on April 30, 1989. They have two children together.

Watching his frantic mannerisms and immediate changes in personality, some have speculated that Williams is affected by bipolar disorder, but this has never been confirmed. A more likely explanation for his remarkable creativity and intense impulsive humor may be the reason that he is often referred to as "the poster child for ADD," though this statement is often said with a sarcastic edge.

In a 2005 poll to find The Comedian's Comedian, he was voted amongst the top 50 comedy acts ever by fellow comedians and comedy insiders.

Robin Williams, along with other celebrities, has performed for U.S. Army troops stationed in Iraq. Robin himself has done so for 3 years.