An American dramatist, Horovitz has written more than 50 produced plays, many of which have been translated and performed in more than 30 languages worldwide .

Among his best-known plays are Line (which opened in 1974 and is now in its 35th year of continuous performance at off-off-Broadway's 13th Street Repertory Theatre), Park Your Car in Harvard Yard, The Primary English Class, The Widow's Blind Date, What Strong Fences Make, and The Indian Wants the Bronx, for which he won the Obie Award for Best Play, and which featured two still-undiscovered future film stars: John Cazale and Al Pacino.

Horovitz is artistic director of the Gloucester Stage Company in Gloucester, Massachusetts, which he founded in 1979. He founded The New York Playwrights Lab in 1975, and still serves as the NYPL's Artistic Director.

Horovitz had a long-term friendship with Irish playwright Samuel Beckett and often found in Beckett a thematic and stylistic model and inspiration for his own work.

His screenplay for the 1982 film Author! Author!, starring Al Pacino, is a largely autobiographical account of a playwright dealing with the stress of having his play produced on Broadway while trying to raise a large family. Other Horovitz-penned films include the award-winning Sunshine, co-written with Istvan Szabo (European Academy Award - Best Screenplay), 3 Weeks After Paradise (which he directed and in which he starred), James Dean, an award-winning biography of the actor, and The Strawberry Statement (Prix du Jury, Cannes Film festival, 1970), a movie adapted from a journalistic novel by James Simon Kunen that deals with the student political unrest of the 1960s.

Horovitz was born in Wakefield, Massachusetts, the son of Hazel Rose (née Solberg) and Julius Charles Horovitz, a lawyer. He has five children: film producer Rachael Horovitz (About Schmidt) (born 1961), television producer-director Matthew Horovitz (NBA network) (born 1964), Adam Horovitz of the Beastie Boys (born 1966) with Doris Keefe, and twins Hannah and Oliver Horovitz (born 1985) with Gillian Adams (born 1955), to whom he is currently married.

He has won numerous awards for his work, including two Obies, the Drama Desk Award, The Sony Radio Academy Award (for Man In Snow on BBC-Radio 4), an Award in Literature from The American Academy of Arts and Letters, The Governor of Massachusetts' Leadership Award, and many others.