Fleishaker was first cast in Troma's 1988 action-comedy Troma's War as an extra, where he played a member of a violent terrorist sect. Lloyd Kaufman, the film's director, was amused by Fleishaker's screen presence as an obese guerrilla soldier and ended up giving him more screen time throughout the film. Following Troma's War, Fleishaker struck up an ongoing partnership with Troma, having acted in every Kaufman-directed film from 1989 to 2006. These parts have ranged from background parts and short cameos to much larger supporting roles, such as The Toxic Avenger's sidekick Lardass in Citizen Toxie: The Toxic Avenger IV.
Outside of their films, Fleishaker became well-known to Troma fans for his numerous appearances in the company's many promotional videos and shorts, where he assumed the role of Troma co-president Michael Herz, who typically prefers to stay out of the public eye. Fleishaker's appearances as Herz were so frequent, many fans have continued to mistake him for the real Michael Herz.
In addition to his work with Troma, Fleishaker appeared at least twelve times as a bit player on The Late Show with David Letterman during the 1990s, acting in various comedy sketches.
In July 2012, Fleishaker was rushed to the hospital where he was diagnosed as suffering from kidney, heart and lung failure, weighing in at nearly 525 pounds. He was later transferred to a physical rehabilitation center where he eventually lost 160 pounds though still couldn't retain the ability to walk. On June 3, 2014, a GoFundMe account was opened in Fleishaker's name, aiming to raise $50,000 to help cover his ongoing medical costs and continued treatment. The fundraiser failed, raising only $931 since the fundraiser opened.
Sometime around late May 2016, it was reported via Lloyd Kaufman's Facebook page that Fleishaker had passed away at the age of 62 at Montefiore Medical Center in the Bronx of a possible heart attack.  
- I Spill Your Guts (2012) as Dave the Landlord
- Poultrygeist: Night of the Chicken Dead (2006) as Jarred (credited as Mega Herz)
- Tales from the Crapper (2004) as Michael Herz/500 Pound Candygramm
- UnConventional (2004) as Himself
- Parts of the Family (2003) as Fat Guy Watching Television (uncredited)
- Zombiegeddon (2003) as Caller #1 (voice only)
- Captain Bill and the Rockin' Buccaneers (2001) as Mr. McDoohan
- Citizen Toxie: The Toxic Avenger IV (2000) as Chester/Lardass (credited as Joseph Isaac Fleishaker)
- Terror Firmer (1999) as Jacob Gelman
- Tromeo and Juliet (1996) as 1-900-HOT-HUNK
- Sgt. Kabukiman N.Y.P.D. (1991) as Josephs
- The Toxic Avenger Part III: The Last Temptation of Toxie (1989) as Apocalypse Inc. Executive
- The Toxic Avenger Part II (1989) as Apocalypse Inc. Executive
- Troma's War (1988) as Specially Trained Terrorist
- Troma's Edge TV (2000) as Michael Herz/Lord Fartacus
- Late Show with David Letterman - Himself/Various characters
- The Tromaville Café (1997) as Michael Herz/God