Williams, whose full name was Richard Treat Williams, lived in Manchester Center in southern Vermont, police said.
His agent, Barry McPherson, also confirmed the actor’s death.
“I’m just devastated. He was the nicest guy. He was so talented,” McPherson told People magazine.
“He was an actor’s actor.
“Filmmakers loved him. He’s been the heart of ... Hollywood since the late 1970s.”
The Connecticut-born Williams made his movie debut in 1975 as a police officer in the movie Deadly Hero and went on to appear in more than 120 TV and film roles, including the movies The Eagle Has Landed, Prince of the City and Once Upon a Time in America.
He was nominated for a Golden Globe Award for his role as hippie leader George Berger in the 1979 movie version of the hit musical Hair.
He appeared in dozens of television shows but was perhaps best known for his starring role from 2002 to 2006 in Everwood as Dr Andrew Brown, a widowed brain surgeon from Manhattan who moves with his two children to the Colorado mountain town of that name.
Williams also had a recurring role as Lenny Ross on the TV show Blue Bloods.
Williams’ stage appearances included Broadway shows, including Grease and Pirates of Penzance.
Colleagues and friends praised Williams as kind, generous and creative.
“Treat Williams was a passionate, adventurous, creative man,” actor Wendell Pierce tweeted. “In a short period of time, he quickly befriended me & his adventurous spirit was infectious. We worked on just 1 film together but occasionally connected over the years. Kind and generous with advice and support. RIP.”
Justine Williams, a writer, director and producer, tweeted that Williams was “the best”. Actor James Woods said, “I really loved him and am devastated that he’s gone.”