His film roles included First Blood (1982), Gorky Park (1983), Silverado (1985), Cocoon (1985), F/X (1986), Presumed Innocent (1990), Tommy Boy (1995), Romeo + Juliet (1996), Ratatouille (2007), and Knight of Cups (2015).

Dennehy's final film was Driveways (2019), in which he plays a veteran of the Korean War, living alone, who befriends a young, shy boy who has come with his mother to clean out his deceased aunt's hoarded home.

[9][10] The family relocated to Long Island, New York, where Dennehy attended Chaminade High School in the village of Mineola.

[12] While acting in regional theater he supported his family by working blue-collar jobs including driving a taxi and bartending.

[19] His earlier films included several comedies, like Semi-Tough (1977) with Burt Reynolds (in which he portrayed a pro football player), Foul Play (1978) with Chevy Chase, and 10 (1979) with Dudley Moore (as a Manzanillo bartender).

Dennehy gradually became a valuable character actor but also achieved leading-man status in the thriller Best Seller (1987) co-starring James Woods.

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.

Dennehy starred as Clarence Darrow in Alleged, a film based on the Scopes Monkey Trial, the famous court battle over the teaching of evolution in American public schools.

[20] Dennehy's early professional acting career included small guest roles in such 1970s and 1980s series as Kojak, Lou Grant, Dallas, Dynasty, and Hunter.

Dennehy portrayed Sergeant Ned T. "Frozen Chosen" Coleman in the television movie A Rumor of War (1980) opposite Brad Davis.

He continued to appear in such high-profile television films as Skokie (1981), Split Image (1982), Day One (1989), and A Killing in a Small Town (1990) opposite Barbara Hershey.

Dennehy had a lead role as fire chief/celebrity dad Leslie "Buddy" Krebs in the short-lived 1982 series Star of the Family.

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

][citation needed] 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 2015 Dennehy co-starred in the Amazon Studios pilot Cocked with Jason Lee, Dreama Walker, Diora Baird, and Sam Trammell.

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.

On stage, Dennehy frequently performed in the Chicago theater world, and made his Broadway debut in 1995 in Brian Friel's Translations.

[27] 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.