Brian Manion Dennehy was born on July 9, 1938, in Bridgeport, Connecticut, the son of Hannah (Manion) and Edward Dennehy, a wire service editor for the Associated Press. He had two brothers, Michael and Edward. He was of Irish ancestry and was raised Catholic. The family relocated to Long Island, New York, where Dennehy attended Chaminade High School in the village of Mineola.

He entered Columbia University in New York City on a football scholarship in the fall of 1956. He interrupted his college education to spend five years in the U.S. Marines. He was stationed in the U.S., Japan, and Korea. He returned to Columbia in 1960 and graduated in 1965 with a B.A. in history. While acting in regional theater he supported his family by working blue-collar jobs, driving a taxi and bartending. He hated his brief stint as a stockbroker for Merrill Lynch in their Manhattan office in the mid 1970s. He later described how working odd hours allowed him to attend matinee theater performances that provided his acting education: “I never went to acting school–I was a truck driver and I used to go see everything I could see–Wednesday afternoons.” In the 1970s, stage performances in New York led to television and film work.

His earlier films included several comedies, like Semi-Tough with Burt Reynolds (in which he portrayed a pro football player), Foul Play with Chevy Chase, and 10 with Dudley Moore (as a Manzanillo bartender). He later portrayed a corrupt sheriff in the western Silverado and an alien in Cocoon, both released in 1985.

Dennehy married for the first time while in the Marines in the early 1960s. Before he finished college he and his first wife had three daughters. Two of them became actresses, including Elizabeth Dennehy. After his first marriage ended in divorce in 1987, he married Jennifer Arnott, an Australian, in 1988, and they had two children, a boy and a girl.

He went on to star as Harrison in the Australian film The Man from Snowy River II in 1988.

Dennehy enlisted in the United States Marine Corps from 1958 to 1963, including a brief stint on Okinawa. In a 1989 interview, he described being hurt in combat and in 1993, he told an interviewer he had served in Vietnam. In 1999, he apologized for misrepresenting his military record.

Dennehy was nominated for Emmy Awards six times for his television movies. In 1992, he was nominated for Outstanding Lead Actor in a Miniseries or TV Movie for his performance as John Wayne Gacy in To Catch a Killer, and he was nominated that same year in a different category, Outstanding Supporting Actor in a Miniseries or TV Movie, for The Burden of Proof. Dennehy’s other Emmy nominations were for his work in A Killing in a Small Town, Murder in the Heartland (1993) and his work in the Showtime cable TV movie Our Fathers (2005), which was about the Roman Catholic Church sex abuse scandal. In 2000, Dennehy was nominated for an Emmy for Outstanding Lead Actor in a Miniseries or TV Movie for a television presentation for his performance as Willy Loman in Arthur Miller’s Death of a Salesman which he had performed on Broadway. While not gaining the actor an Emmy win, the performance did, however, win him a Golden Globe Award. He also appeared as a recurring character in the NBC sitcom Just Shoot Me!.

In fall 1992, he played the lead role of Hickey in Robert Falls’s production of Eugene O’Neill’s The Iceman Cometh at the Abbey Theatre in Dublin.

One of his most well-known roles came in the 1995 Chris Farley-David Spade comedy Tommy Boy as Big Tom Callahan. He also was reunited with his 10 co-star Bo Derek in Tommy Boy, in which she played his wife.

On stage, Dennehy frequently performed in the Chicago theater world, and made his Broadway debut in 1995 in Brian Friel’s Translations. In 1999, he was the first male performer to be voted the Sarah Siddons Award for his work in Chicago theater. He made a return to Broadway in 2007 as Matthew Harrison Brady in Inherit the Wind opposite Christopher Plummer, then returned again opposite Carla Gugino in a 2009 revival of Eugene O’Neill’s Desire Under the Elms.

A cartoon version of Dennehy appeared in the 1999 film South Park: Bigger, Longer & Uncut as well as in an episode of The Simpsons.

Dennehy won two Tony Awards, both times for Best Lead Actor in a Play. His first win was for Death of a Salesman (for which he also won a Laurence Olivier Award for the production’s London run), in 1999, and the second was for Eugene O’Neill’s Long Day’s Journey into Night in 2003. Both productions were directed by Robert Falls and were originally produced at the Goodman Theatre company in Chicago, Illinois. His acting in the “Salesman” was called “the performance of Dennehy’s career.”

In January 2007, he starred in the episode “Scheherazade” of Law & Order: Special Victims Unit as a retired criminal who wants to reconnect with his daughter and admit his crimes before dying of a terminal disease thus eventually clearing a wrongfully imprisoned inmate. In April 2008, Dennehy guest-starred as a Teamster boss in “Sandwich Day”, an episode of the TV series 30 Rock. He guest-starred in a 2009 episode of Rules of Engagement as the father of the main character, Jeff. Dennehy starred as Elizabeth Keen’s grandfather on the NBC series The Blacklist.

Dennehy had a voice role in the animated movie Ratatouille as Django, father of the rat chef Remy. He appeared as the superior officer of Robert De Niro and Al Pacino in the 2008 cop drama Righteous Kill and as the father of Russell Crowe in the 2010 suspense film The Next Three Days.

In 2008, Dennehy appeared at the Stratford Shakespeare Festival, in Stratford, Ontario, Canada, appearing in All’s Well That Ends Well as the King of France, and a double bill of plays, Samuel Beckett’s Krapp’s Last Tape and Eugene O’Neill’s Hughie, where Dennehy reprised the role of Erie Smith. In 2010, he was inducted into the American Theatre Hall of Fame. In December 2010, he returned to Ireland to play Bull McCabe in the Olympia Theatre of Dublin’s stage version of John B. Keane’s The Field.

In 2011, Dennehy returned to the Stratford Shakespeare Festival in the role of Sir Toby Belch in Shakespeare’s Twelfth Night. He also played Max in Harold Pinter’s The Homecoming, the first Pinter work to be produced there.

In April through June 2012, he played the role of Larry Slade in the Eugene O’Neill play The Iceman Cometh at the Goodman Theatre in Chicago, which he reprised in 2015 when the production, with most of the Goodman Theater production cast, was revived at the BAM Harvey Theater in Brooklyn, New York, New York.

Dennehy died on April 15, 2020, of cardiac arrest due to sepsis in New Haven, Connecticut. He was surrounded by Arnott and his five children.