Haggerty was cast in a small non-speaking role as a Bodybuilder in the 1964 film Muscle Beach Party and also as a Bodybuilder in Girl Happy. These were followed by appearances in various biker and wildlife films such as Easy Rider, Angels Die Hard, The Adventures of Frontier Fremont, and Terror Out of the Sky.

Haggerty is best known for portraying the character Grizzly Adams in the title role of the 1974 Sunn Classic Pictures feature, The Life and Times of Grizzly Adams. From this feature film evolved the NBC television series of the same name which ran from 1977 to 1978, and Haggerty became known to movie-goers for his portrayal of nature-loving James Capen "Grizzly" Adams.

Haggerty lived on a small ranch in Malibu Canyon with an assortment of wild animals that he had tamed at birth or rescued from injury. In 1977, his beard caught fire from a careless patron carrying a flaming cocktail. Trying to put the flames out, Haggerty received third-degree burns on his arms. In 1991, Haggerty was again hospitalized after a motorcycle accident left him in a coma, recovering with no side effects.

Haggerty starred in the TV miniseries Condominium (1980), which also starred Barbara Eden, Ralph Bellamy and Stuart Whitman. Haggerty played a hydraulics expert trying to warn residents that their Florida condos were about to be demolished in a forthcoming hurricane. He was also on "Charlie's Angels", season 5 episode 5 called " Waikiki Angels" as 'Bo Thompson' that aired January 4 1981. He guest starred on The Love Boat in 1983 ("World's Greatest Kisser"). He cameoed as an attorney in Terror Night (1987) with John Ireland and Cameron Mitchell, starred in Night Wars (1988) as a Vietnam veteran who is a Psychologist dealing with nightmares of his fellow veterans, and appeared in horror films such as Elves and the Linda Blair film The Chilling in 1989. In Big Stan (2007), he played Tubby, and appeared as a lumberjack foreman in Axe Giant: The Wrath of Paul Bunyan (2013). Haggerty has also done several voice-overs and can also be seen in music videos by Hank Williams, Jr. and Rogues of the Empire.

Haggerty appeared briefly in David Carradine's film Americana and provided a fighting dog for the production. In the film, he not only played the role of the dog's trainer but also assisted in set design and the restoration of a broken down carousel, which figured prominently in the film. He assisted in building the motorcycles featured in the film Easy Rider, and had a bit part as a hippie in that film, as well as stunt work and supporting roles in numerous low-budget biker films of the era. He also starred in the 1989 film Spirit of the Eagle.

Haggerty played lead roles in the films Repo Jake (1990), Elves (1990), Grizzly Mountain (1995), and Escape to Grizzly Mountain (2000). He was also featured as a character in Al Franken's novel, Why Not Me?.

His experience with animals also brought him work as an animal trainer and handler in films produced by Walt Disney Studios. Additionally, Haggerty also worked as a stuntman on the television series Tarzan, and as set builder on various other projects. He directed white tigers, wolverines, eagles and wild boar in When the North Wind Blows and worked with bears, foxes and hawks in the 1997 film Grizzly Mountain.

Haggerty was married twice. He married Diane Rooker in 1959. They were 17 and had a wedding chapel in Las Vegas called Silver Slipper Hotel. Haggerty and Diane had two daughters, Tracey and Tammy, and one son, Don. They divorced in 1984, after which Haggerty married Samantha Hilton. Haggerty and Samantha had two sons named Dylan and Cody and one daughter named Megan. They were married until Samantha's death following a motorcycle accident on August 10, 2008.

Also played a role in Dead In 5 Heartbeats, which is on Amazon. Circa 2013. Imdb 7/10.

Haggerty was diagnosed with spinal cancer after undergoing back surgery, when a tumor on his spine was discovered in August 2015. He died of spinal cancer on January 15, 2016, in Burbank, California.