In 1966, Chalfant married Henry Chalfant, a Photographer and documentary filmmaker. They have a son, David Chalfant, who was the bass player for the folk-rock band, The Nields, and a daughter, Andromache, a set designer in New York.

Chalfant worked as a Production Coordinator at Playwrights Horizons in the mid-1970s, beginning with Demons: A Possession by Robert Karmon. She made her Off-Broadway acting debut in Cowboy Pictures in June 1974. She has since appeared in over three dozen Off-Broadway productions. In 2015, she appeared in the Women's Project Theater production of Dear Elizabeth by Sarah Ruhl and as Rose Kennedy in the Nora's Playhouse production of Rose by Laurence Leamer.

Chalfant was nominated for the 1993 Tony Award as Best Actress (Featured Role - Play) in Tony Kushner's Angels in America: Millennium Approaches. She earned the Outer Circle Critics, Drama Desk, Obie and Lucille Lortel awards for her performance as Vivian Bearing in Margaret Edson's Pulitzer Prize-winning play Wit in 1998; she shaved her head for the role. During her work with Wit, she incorporated her experiences dealing with terminal cancer of her half-brother, Alan Palmer, who died in 1998.

For her 2003 performance in Alan Bennett's Talking Heads, Chalfant won a second Obie award. In 2009, Chalfant performed in The People Speak, a documentary feature film utilizing dramatic and musical performances of the letters, diaries, and speeches of everyday Americans, based on Historian Howard Zinn's A People's History of the United States.