He worked as a stockbroker at the same firm as Martha Stewart. The two remained good friends.
He served in the United States Marine Corps, but contrary to earlier press statements, he never saw combat and was not wounded.
His younger brother is an FBI agent.
His football coach and English teacher at Chaminade High School steered him toward both sports and acting. While there, he played both the fight and the title role in "Macbeth."
He first began acting in series and films at the age of 39.
He described his co-star Saoirse Ronan as the most talented actor he had ever worked with.
He was the father of two adopted children: Cormack (b. 1993) and Sarah (b. 1995).
While performing on Broadway in "Death of a Salesman" and "Long Day's Journey into Night," many of his fans wanted to meet him after the show, and producers had to hire extra security to control the crowd near the stage door.
He won two Tony Awards for Best Actor (Play) with only two nominations: in 1999 for the revival of Arthur Miller's "Death of a Salesman," and in 2003 for the revival of Eugene O'Neill's "Long Day's Journey into Night."
He lived in Woodstock, Connecticut.
He began his stage career appearing in both summer stock and Off-Broadway productions.
He was the only actor to appear in both Presumed Innocent (1990) and its made-for-television sequel The Burden of Proof (1992), playing a different role in each.
On March 14, 1999, three performances of the Broadway production of "Death of a Salesman" had to be canceled because Dennehy could not sleep and was not feeling well. After tests at the hospital, he was diagnosed with hypertension.
He was the primary consideration for the original Hannibal Lecter role in Manhunter (1986), until he recommended Scotsman Brian Cox to director Michael Mann.
He stated that his favorite film was On the Waterfront (1954).
He grew up on Macatee Pl in Mineola, NY.
He graduated with a BFA in Theatre / Performing Arts from Columbia University in New York.
He once owned a house in Santa Fe, New Mexico, and a small cottage in Ireland.
On "The Tonight Show," he often joked about being confused with Charles Durning. In South Park: Bigger, Longer & Uncut (1999), as the children sing an admiring song about figure skater Brian Boitano, they are thrown off when Brian Dennehy suddenly appears, having confused the name with his own.
He was the father of Kathleen Dennehy and Elizabeth Dennehy.
He was the son of Hannah (Manion) and Edward Joseph Dennehy, a news services editor for the Associated Press. His father's grandparents were Irish immigrants, and his mother was also of Irish descent.
He took part in OCLC's "Geek the Library" advocacy campaign and appeared on a poster that read "I geek schooners."
He was honored with the 1999 Joseph Jefferson Award for Actor in a Principal Role in a Play for "Death of a Salesman" at the Goodman Theatre in Chicago, Illinois.
He was honored with the 2012 Equity Joseph Jefferson Award for Actor in a Supporting Role in a Play for "The Iceman Cometh" at the Goodman Theatre in Chicago, Illinois.
He appeared in a limited run of 'Death of a Salesman' at the Lyric Theatre in London.
Acting at the Stratford Shakespeare Festival in Ontario, Canada. Playing the "King of France" in William Shakespeare's "All's Well that Ends Well" from June 19 to August 23. Playing the roles of "Erie Smith" and "Krapp" in the double bill of "Hughie" (by Eugene O'Neill) and "Krapp's Last Tape" (by Samuel Beckett) from June 18 to August 31.
Chicago, IL: Performing the role of Larry Slade in the Goodman Theatre production of "The Iceman Cometh."
He was honored with the 2000 Back Stage Garland Award for his performance in "Death of a Salesman" at the Ahmanson Theatre in Los Angeles, California.
He was a member of the Sigma Chi Fraternity.
He was nominated for the 2010 Joseph Jefferson Award for Actor in a Principal Role in a Play for "Hughie/Krapp's Last Tape" at the Goodman Theatre in Chicago, Illinois.
Stratford, ON, Canada: Stratford Shakespeare Festival - 'Twelfth Night' and 'The Homecoming.'
He was nominated for the 1987 Joseph Jefferson Award for a Leading Role in a Play for "Galileo" at the Goodman Theatre in Chicago, Illinois.
He was honored with the 1980 Drama-Logue Award for Outstanding Performance for the play "Says I, Says He," at the Mark Taper Forum Theatre in Los Angeles, California.
He appeared on Broadway in "Long Day's Journey Into Night."
In 2021, he was inducted into the Derby [Connecticut] Hall of Fame.