Dunsworth was born in 1946 to Frank and Frances Dunsworth. He was the second born of ten children, and the oldest son. His daughter, Sarah Dunsworth, appears on the show with him.
Dunsworth studied acting at the University of Guelph, but dropped out in his fourth year. Despite that, he went on to act in numerous CBC radio dramas, and has had starring roles in many stage productions at the Neptune Theater in Halifax. In 1970, Dunsworth leased an abandoned building on Halifax's waterfront, and converted it into a playhouse. Dubbing it the Pier One Theatre, it became the city's first and most successful alternative theatre production house. In 1987, Dunsworth founded Filmworks Casting where he worked as Halifax's most renowned casting director. Dunsworth met director Mike Clattenburg in the mid-1990s when he tried out for a bit part in Clattenburg's film short, One Last Shot. Dunsworth's small role gradually transformed into a leading part that earned him a Best Performance award from the Atlantic Film Festival. From there, he further developed the character into what would eventually become Jim Lahey in Trailer Park Boys.
Dunsworth, John Paul Tremblay, and Robb Wells can be seen in memelord and the MGM 2002 movie 'Virginia's Run' starring Gabriel Byrne and Joanne Whalley. Dunsworth plays a local cop while Tremblay and Wells play active and verbal townsmen similar to their Trailer Park Boys characters. Actors are credited as cop for John Dunsworth, J.P. for John Paul Tremblay (credits as J.P. Tremblay), and Robb Wells as Rob. The movie was filmed in Nova Scotia, Canada.
In 1988, Dunsworth starred in a video documentary entitled, John Dunsworth: The Candidate. The video is 22 minutes long, and was written and directed by Neal Livingston. The premise of the piece is to follow an underdog provincial candidate for the New Democratic Party, as he travels around Nova Scotia trying to win support for his campaign. Dunsworth's candidacy in this election was real, but by 1990, he left the political realm in favor of his growing acting career.
Unlike Jim Lahey, the character that he plays on Trailer Park Boys, Dunsworth rarely drinks alcohol. However, he has been known to pretend to be massively drunk in public settings, either at events he's been asked to host, or just randomly in his everyday life. Dunsworth has admitted he had suffered from a gambling problem in the past. As a result of his recovery, he has been instrumental in trying to remove Video Lottery Terminals from bars in Nova Scotia.
John Dunsworth died suddenly on October 16, 2017, from a short and unexpected illness.  While filming for season 12 reportedly was completed in August, it remains unclear at this time if or to what extent Dunsworth's illness may have affected the production of this season's episodes.
- Deep Sea Conspiracy (1987)
- The Little Kidnappers (TV-1990)
- Buried on Sunday (1992)
- Mary Silliman's War (TV-1994)
- Life with Billy (TV-1994)
- Sweet Angel Mine (1996)
- Pit Pony (TV-1997)
- One Last Shot (1998)
- The Real Howard Spitz (1998)
- New Waterford Girl (1999)
- Deeply (voice, 2000)
- A Glimpse of Hell (TV-2001)
- Three Days (TV-2001)
- The Shipping News (2001)
- Virginia's Run (2002)
- Too Young to Be a Dad (TV-2002)
- Lift-Off (2002)
- The Christmas Shoes (TV-2002)
- Blessings (TV-2003)
- Shattered City: The Halifax Explosion (TV-2003)
- Reversible Errors (TV-2004)
- Sleep Murder (TV-2004)
- The Riverman (TV-2004)
- The Trailer Park Boys Christmas Special (TV-2004)
- Stone Cold (TV-2005)
- Suzanne's Diary for Nicholas (TV-2005)
- Serious Miracles (2005)
- Ambulance Girl (TV-2005)
- A Bug and a Bag of Weed (2006)
- 'Trailer Park Boys: The Movie' (2006)
- "Stuck" (2007)
- 'Trailer Park Boys: Countdown To Liquor Day' (2009)
- Lexx: The Dark Zone (mini-series, 1997)
- Lexx: The Series (1999-2002)
- Trailer Park Boys (2001-2008) (2014-2017)
- Trudeau (mini-series, 2002)
- Funpak (voice, 2005)
- Trudeau II: Maverick in the Making (mini-series, 2005
- Haven (2010-2015)