Hearn studied philosophy at Southwestern University before he embarked on a career in the theater, training for the stage with legendary actress turned acting coach Irene Dailey. Most of Hearn's early performances were in traditional productions at the New York Shakespeare Festival and theaters at Lincoln Center.

He first garnered a notice as John Dickinson in the acclaimed 1969 award-winning musical 1776, and as Liv Ullmann's leading man in the musical version of I Remember Mama (1979). Hearn grabbed headlines later that year, when he replaced Len Cariou in the title role of Stephen Sondheim's dark musical Sweeney Todd on Broadway, opposite Dorothy Loudon as a woman who bakes his character's murder victims into pies. Hearn and the show's original star, Angela Lansbury, later headed the show's touring company, then reprised their roles for a Showtime production of the musical, which brought him an Emmy Award for his chilling portrayal of the "Demon Barber of Fleet Street".

Hearn and Lansbury remained friends, and the actress invited him to guest star on several episodes of her long-running CBS sleuth series Murder, She Wrote in the early 1990s.

His recordings include Sweeney Todd Live at the New York Philharmonic, Mack & Mabel (1988 London Concert Cast), I Remember Mama (1985 Studio Cast), Follies in Concert (1985 Live Performance), and A Stephen Sondheim Evening (1983 Concert Cast).

Hearn's spouses include Susan Babel (? - ?) (divorced); Mary Harrell (? - 1962) (divorced), 1 child (1962); Dixie Carter (1977 - 1979) (divorced); and Leslie Simons (1985 - present) 2 children (1997, 1999).

Preceded by

Tommy Tune

for My One and Only

Tony Award for Best Leading Actor in a Musical

for La Cage Aux Folles

Succeeded by

George Rose

for The Mystery of Edwin Drood

Preceded by

Jarrod Emick

for Damn Yankees

Tony Award for Best Featured Actor in a Musical

for Sunset Boulevard

Succeeded by

Wilson Jermaine Heredia

for Rent