Concurrent with his appearances on The Addams Family, Cassidy began doing character voices on a recurring basis for the Hanna-Barbera Studios, culminating in the role of Frankenstein Jr. in Frankenstein Jr. and The Impossibles series. He was the voice of the hero in the Chuck Menville pixillated short film "Blaze Glory (1969)" in which his already deep voice was enhanced with a deep reverb & echo to give the character an exaggerated super-hero sound. Cassidy also voiced Ben Grimm aka The Thing in the 1978 animated Fantastic Four. Cassidy went on to perform the roars and growls for Godzilla in the 1979 cartoon series that Hanna Barbera co-produced with Toho. His was the basis for the sinister voice of Black Manta, as well as Brainiac and several others on Super Friends.

After The Addams Family, Cassidy began to add more voice-over work to his résumé; in that acting field, most notably, he narrated the opening of the TV series The Incredible Hulk. Cassidy also provided the Hulk's growls and roars. He detested being compared or confused with acromegalic actor Richard Kiel, who played "Jaws," the assassin with stainless steel teeth, in two James Bond films and in the 1999 film Inspector Gadget.[citation needed]

In deleted scenes from the original Battlestar Galactica TV pilot movie, "Saga of a Star World" (on the DVD collection Battlestar Galactica: The Complete Epic Series ), Cassidy can be heard providing temporary voice tracks of the Cylon Imperious Leader, before actor Patrick Macnee was contracted to voice the character.

Other film work included his appearances in Butch Cassidy and the Sundance Kid, Mackenna's Gold, Goin' Coconuts, The Last Remake of Beau Geste, Poor Pretty Eddie, Harry and Walter Go to New York, The Slams, The Limit, and Charcoal Black. He also co-wrote the screenplay of 1973's The Harrad Experiment, in which he made a brief appearance.

Cassidy suffered from health issues as he grew older, and died at the age of 46 from complications following open-heart surgery. Fellow actor Sandra Martinez assisted and took care of Ted during his final years. Cassidy's remains were cremated, and later buried in the backyard of his Woodland Hills home.