[1][7] He then returned to Grease on Broadway in the lead role of Danny Zuko for three years, later saying of the experience, "I had grown up learning all of the songs from West Side Story, so I was aware of what a big deal 'Broadway' was.

He received positive notices the following year for his portrayal of Michael Brick—a squeaky-voiced private detective—in The Ritz,[8] a farcical comedy based on the play of the same name, and next appeared in a small part in the British war film The Eagle Has Landed (1976).

Williams came to worldwide attention in 1979 when he starred as George Berger—a leading member of a gang of flower children—in the Miloš Forman film Hair, based on the 1967 musical.

Writing for the Sioux City Journal, critic Bob Thomas called it "a rare flight of creative imagination that widens the dimensions of the movie musical" and believed that Williams' performance "could not be better".

[9] In her mixed review for The New York Times, Janet Maslin wrote, "As his name might indicate, Treat Williams is one of the better things Hair has to offer … [he is] is the only one of the players who really suggests the spirit of euphoria upon which the original [stage production] meant to capitalize".

Throughout the next decade, Williams appeared in a variety of supporting and leading film roles, such as the adventure drama The Pursuit of D. B. Cooper (1981), in which he played the titular aircraft hijacker; the Sergio Leone crime epic Once Upon A Time In America (1984); action-thriller Flashpoint (1984); Peter Medak's The Men's Club (1986); and the cult horror-comedy Dead Heat (1988).

[12] In a retrospective review of the film published by Empire in 2000, Simon Braud wrote: It's doubtful whether a better performance was committed to celluloid in 1981 than Treat Williams' portrayal of the tortured Danny Ciello.

In a staggering feat of acting prowess, Williams essays a fundamentally good, yet deeply flawed, human being disintegrating under intolerable pressure with rare courage and intensity.

Writing for the Chicago Sun-Times, Roger Ebert said he found the film "smashingly entertaining", adding, "Williams [is] implacably evil … and also slick and oily in the best pulp tradition".

That same year, his "messianic" work in The Late Shift,[19] an HBO television film in which he portrayed real-life talent agent Michael Ovitz, was roundly praised by critics and earned him an Emmy Award nomination for Outstanding Supporting Actor.

He returned to the stage in 1999, earning critical acclaim for his work as Portuguese fisherman Manuel in the off-Broadway production of Captains Courageous, the Musical.

[22][23] Next, he appeared as Buddy in the Broadway revival of Stephen Sondheim's Follies (April–July 2001), and co-starred in the Woody Allen-directed satire Hollywood Ending (2002), with Elvis Mitchell of The New York Times calling his portrayal of film studio boss Hal Jaeger "frightening and impressive".

[24] His next major film role was that of egotistical FBI boss Walter Collins in the big-budget action comedy Miss Congeniality 2: Armed and Fabulous (2005).

Between 2002 and 2006, Williams played the lead role of Dr. Andrew Brown on the WB's Everwood, a drama series about a widowed neurosurgeon moving from New York City to Colorado with his two children.

[27] Between July and November 2006, he made several appearances on the first season of ABC's Brothers & Sisters, playing David Morton, a friend and potential suitor of protagonist Nora Walker.

[35] Williams played the principal role of Mick O'Brien on the Hallmark series Chesapeake Shores from 2016 until its final episode, which aired in October 2022.

[36] His other television appearances during the 2010s–2020s included recurring roles on White Collar (2012–2013) as Samuel Phelps; Chicago Fire (2013–2018) as Benny Severide; and Blue Bloods (2016–2023) as Lenny Ross, the former police partner of character Frank Reagan.

[40] His portrayal of Dr. "Andy" Brown on the WB's Everwood (2002–2006) earned him two nominations for the Screen Actors Guild Award for Outstanding Performance in a Drama Series (2003, 2004).

Williams lived in Park City, Utah, and Manchester Center, Vermont,[5][41] with his wife, actress and producer Pam Van Sant (m. 1988), and their two children, Gill and Ellie.

Williams was open about his struggles with drug addiction during the 1980s, which he believed hampered his career at a time when he was being called an "up-and-coming Pacino or De Niro".

[47][48][49][50] On March 8, 2024, 35-year-old Ryan Koss, the driver of the car that hit Williams, pleaded guilty to a reduced charge of negligent driving resulting in death.