Williams was born in Rowayton, Connecticut, the son of Marian (née Andrew), an antiques dealer, and Richard Norman Williams, a corporate executive. His maternal great-great-great-grandfather was Senator William Henry Barnum of Connecticut, a third cousin of the showman P. T. Barnum, and a distant relative was Robert Treat Paine, who was a signatory to the Declaration of Independence.
Williams played football in high school. He graduated from the Kent School in Connecticut and Franklin and Marshall College.
Williams made his film debut in the 1975 thriller film Deadly Hero. The following year he played a supporting actor in The Ritz, a squeaky-voiced private detective looking for his suspect in a gay bathhouse. He came to world attention in 1979, when he starred as George Berger in the Miloš Forman film Hair, which was based on the 1967 Broadway musical. Williams was nominated for a Golden Globe Award for his role in the film. He was featured in the February 1980 edition of Playgirl magazine. He has gone on to appear in over 75 films and several television series. Notable films include: 1941 (1979), Once Upon A Time In America (1984), Dead Heat (1988), Things to Do in Denver When You're Dead (1995), and Deep Rising (1998).
Williams' second Golden Globe nomination was for his starring role in Sidney Lumet's Prince of the City (1981). His third nomination was for his performance as Stanley Kowalski in the television presentation of A Streetcar Named Desire. In 1996, he was nominated for a Best Actor Emmy Award by the Academy of Television Arts & Sciences for his work in The Late Shift, an HBO movie, in which he portrayed agent Michael Ovitz.
In 1996, he played villain Xander Drax in Paramount's big budget comic book adaptation The Phantom, in which Williams' character did his best to take over the world and kill Billy Zane's mysterious superhero.
Williams' career includes numerous stage roles. He won a Drama League Award for his work in the Broadway revival of Stephen Sondheim's Follies, and another for starring in the off-Broadway production of Captains Courageous. Other notable Broadway shows include Grease, the Sherman Brothers' Over Here!, Once in a Lifetime, Pirates of Penzance and Love Letters, and off-Broadway, he has appeared in David Mamet's Oleanna and Oh, Hell (at Lincoln Center), Some Men Need Help and Randy Newman's Maybe I'm Doing It Wrong. He premiered the Los Angeles production of Love Letters and appeared in War Letters at the Canon Theatre in Los Angeles.
Williams played leading role as Dr. Andrew Brown in the WB television series Everwood, about a New York City neurosurgeon who moves his family to Colorado. Although the show's ratings were never spectacular, it won critical acclaim and had a devoted following. Williams received two SAG Award nominations (2003 and 2004) for his role on the show.
Williams has made several guest appearances on the ABC drama Brothers & Sisters as David Morton, a friend and potential suitor of Sally Field's character. He starred in the short-lived series Heartland on TNT as Nathaniel Grant, but the series was canceled due to low ratings. He also starred in a Lifetime movie, Staircase Murders, which aired April 15, 2007.
Williams starred in a Hallmark Hall of Fame movie, Beyond the Blackboard, with his former Everwood co-star, Emily VanCamp. It was first broadcast on CBS on April 24, 2011.
Williams appears in the CBS television pilot Peachtree Lines as Mayor Lincoln Rylan. The serial is an examination of political, social, and cultural issues in Atlanta.
Williams has also worked as a director, winning two festival awards for directing Texan in Showtime's Chanticleer Films series.
Williams currently plays a role as Mick O' Brien in a Hallmark Channel television series called Chesapeake Shores originally airing August of 2016 till current.
Williams has authored a children's book, Air Show!, illustrated by Robert Neubecker, published in 2010 by Disney/Hyperion Books). Target at an audience of children ages three to seven, the book playfully documents an airshow experience with simple text and bold illustrations of such aircraft as a Boeing B-17, a Pitts Special biplane, and the US Navy's Blue Angel F/A-18.
In 1969, Williams' high school football coach, who was also a flight instructor, offered to train him in a Piper Super Cub. At age 21, he was a private pilot. Williams became an FAA instrument-rated commercial pilot with privileges in both single engine and multi-engine airplanes as well as rotorcraft helicopter. He holds a type-rating for Cessna Citation jets. He also is certified as a flight instructor. He has owned a Piper Clipped-wing Cub, Piper Cherokee 180, Piper Seneca II, and a Piper Navajo Chieftain which is used for family travel between homes.
Williams resides in Park City, Utah and Manchester Center, Vermont with his wife, Pam Van Sant, and two children, Gill and Ellie.
In early 2010, videos were posted on YouTube as well as edits made to Williams' Wikipedia page and a Facebook page made which falsely reported his death. These included a graphic and detailed explanation as to his illness and place of demise. In August 2011, Williams responded to these rumors in an interview with Contact Music when speaking of more recent rumors linking him to the Quentin Tarantino film Django Unchained. Regarding the death hoax, Williams stated, "I did see that, and I think those things are dangerous for your family and friends."