Was a professional pilot for a year in the early 1980s.
Graduated from Franklin & Marshall College in 1973
The nickname "Treat" comes from one of his maternal relatives (though not a direct ancestor), Robert Treat Paine, whose signature appears on the Declaration of Independence. Through his mother, Treat is also the great-great-great-grandson of William Henry Barnum, a Democratic U.S. Senator from Connecticut, who served from 1876 to 1879.
Certified Flight Instructor, rated in single and multi-engine airplanes and helicopters.
In 2003, completed two weeks of training to qualify for flying jet engine planes.
Had a brief romance with Dana Delany.
Andy Brown, Williams' character on Everwood (2002), was ranked #43 in TV Guide's list of the "50 Greatest TV Dads of All Time" [20 June 2004 issue].
Was invited to join the Academy of Motion Picture Arts and Sciences (AMPAS) in June 2004.
Graduated from Kent School, the same high school that Ted Danson, Peter Farrelly and Seth MacFarlane attended.
Children: Son Gill Williams (b. 1992) and daughter Elinor Williams (b. 1998).
The February 16, 1983, edition of Variety, in the "Production in Italy" section, reports the film "Boogie Woogie" directed by Andrea Frezza, starring Ben Gazzara and Treat Williams, was to begin filming in Italy in March 1983. Lack of funding prohibited completion of the film.