Harnell is originally from Teaneck, New Jersey. He started his career with minimal experience. But in 1993, he voiced his most popular role, Wakko Warner on Animaniacs, and the Secret Squirrel on 2 Stupid Dogs. From 1994 to 1996 he supplied the voice of Sewer Urchin in the animated series The Tick. Meanwhile in 1996 he was Hunter on Road Rovers.
Harnell voiced Joe Tabootie on the Nickelodeon show ChalkZone, Krypto and Smokey on Krypto the Superdog, Crash Bandicoot in Crash Tag Team Racing, Crash of the Titans, and Crash: Mind over Mutant, Lo-Lo in Crash Bandicoot: The Wrath of Cortex, Spyro the Dragon in Spyro: A Hero's Tail and Spyro: Shadow Legacy, Jerry in Totally Spies, Linguni in Pucca, and Doctor Finklestein in The Nightmare Before Christmas's video game spin-offs, as well as in the Kingdom Hearts series replacing the late William Hickey who voiced him in the film. In 2007, he replaced Neil Flynn to voice The Plumber in Ratchet & Clank Future: Tools of Destruction and Ratchet & Clank Future: A Crack in Time. In 2001, he was the singing voice of Buster on Lady and the Tramp II: Scamp's Adventure.
He also voiced Captain Hero on Comedy Central's animated comedy Drawn Together and he also does the voices of Wooton Bassett, and Bennett Charles on the radio drama Adventures in Odyssey, plays the lead role of Finnian Jones for the Lamplighter Theatre Radio Drama, as well as Br'er Rabbit on the Disney TV show House of Mouse as well as in the Disney attraction, Splash Mountain (both at Disneyland and Walt Disney World). He also made a few appearances on Superhuman Samurai Syber-Squad, both on-screen and in voice-over. His voice also made an appearance in NASCAR Rumble, and is credited for in-game commentary in the follow up game Rumble Racing. Jess also replaced Brad Garrett as the voice of Fatso during season 3 of The Spooktacular New Adventures of Casper.
He voiced the characters of Ironhide and Barricade in Michael Bay's Transformers, technically making him the only voice actor to play both an Autobot (Ironhide) and a Decepticon (Barricade). He returned to voice Ironhide once again on Transformers: Revenge of the Fallen.
See his massive credit list here.