Nash made a name for himself in the late 1920s as an impressionist for KHJ, a Los Angeles radio station, on their show, The Merrymakers. He later was employed by the Adohr Milk Company for publicity purposes. Dubbed "Whistling Clarence, the Adohr Bird Man", Nash rode the streets with a team of miniature horses and gave treats to the children. In 1932, Nash happened by the Disney Studio with his team of horses, and decided to leave a copy of his Adohr publicity sheet with the receptionist. As it turns out, his name was recognized from a reprise appearance on The Merrymakers a few days previous, and Walt Disney himself had been impressed by Nash's vocal skills. He was asked to make an informal audition.
Nash went through several of his voices, and Walt Disney happened by when Nash gave his impersonation of a family of ducks. Disney declared Nash perfect for the role of a talking duck in their upcoming animated short, The Wise Little Hen. The duck, of course, was Donald Duck, who Nash went on to voice for almost 50 years, in over 120 shorts and films. The last film to feature Nash's famous voice was 1983's Mickey's Christmas Carol, although he continued to provide Donald's voice for commercials, promos and other miscellaneous material until his death.
Nash's Donald Duck voice was achieved by what is called buccal speech: an alaryngeal form of vocalization which uses the inner cheek to produce sound rather than the larynx. He first discovered it while trying to mimic his pet goat Mary. In his days before Disney, Nash performed in vaudeville shows where he often spoke in a "nervous baby goat" voice.
Donald Duck went on to become one of the most famous cartoon characters in the world, a great part of this due to Nash's voice. The voice is distinctive both for its ducklike quality and the fact that it is often very difficult for anybody to understand, especially when Donald flew into a rage (which happened fairly often). To keep Donald's voice consistent throughout the world, Nash voiced the character in all foreign languages the Disney shorts were translated to (with the aid of the phonetic alphabet), meaning Donald retained his same level of incoherency all across the globe. Mad magazine, in its 1950s comic-strip style satire of Disney characters, featured a "translation" of "Darnold" Duck's "quacky, incomprehensible" voice.
In addition to Donald's voice, Nash also voiced Daisy Duck (in her earliest appearances, when she was little more than a female version of Donald), as well as Donald's nephews Huey, Dewey and Louie. He provided the voice for PJ in the cartoon Bellboy Donald and for a bullfrog in Bambi. Nash also provided the meows of Figaro the kitten in a handful of shorts and did some dog sounds in One Hundred and One Dalmatians. He also voiced Jiminy Cricket for a brief period of time after Cliff Edwards's death in 1971. Nash also provided the meows and screeches for Tom in Tom and Jerry. His last performance in Tom and Jerry came in Mouse in Manhattan (1945), where he was the voice of vicious alley cats.
Nash also provided the Bear's roars and snarls in The Fox and the Hound while Candy Candido did his growls (neither received on-screen credit).
Nash's voice would be impersonated elsewhere in animation, most notably in the Tom and Jerry cartoons directed by William Hanna and Joseph Barbera with three characters Little Quacker (voiced by Red Coffey) and Yakky Doodle (voiced by Jimmy Weldon). As with most Hanna-Barbera productions, these characters used celebrity impersonations, in these cases an impersonation of Clarence Nash's Donald Duck voice. Because both were so similar to Nash's voice they were often mistakenly attributed to Nash. Likewise, contrary to popular belief, he did not perform the duck voice for Rick Dees' "Disco Duck"; it was one of Dees' acquaintances.
Nash appeared as himself in the 1941 movie The Reluctant Dragon, which shows how Disney movies are produced, and was a contestant on a 1954 episode of What's My Line. and a 1964 episode of To Tell the Truth. Nash also appeared as himself in a 1956 episode of Disneyland entitled "A Day in the Life of Donald Duck", in which he interacts with an animated Donald who blames him for his speech problems: the two end up arguing mainly due to Donald's short temper. He was also a guest on a 1976 episode of The Mike Douglas Show. The 1984 special Donald Duck's 50th Birthday included several clips from Disney movies and Disneyland episodes.
When Disney shut down their shorts department in 1962, Nash continued to voice Donald in various projects over the next two decades, notably performing the song "Macho Duck" on the Mickey Mouse Disco album in the 1970s.
In the late 1970s, Nash was known for often taking walks in the neighborhood around Fremont Elementary School in Glendale, California, entertaining children with his Donald Duck voice. As he passed the age of 70, he found the harsh voice increasingly straining on his throat and so limited public performances to groups of children. During recording sessions, he would take frequent breaks and drink plenty of water to avoid overexerting himself.
Nash's final performance before his death was in Mickey's Christmas Carol (1983), which made Donald the only character in the film to be voiced by his original actor. With 50 years having elapsed since the first appearance of Donald Duck in The Wise Little Hen, he and Mel Blanc both retained the distinction of performing the same characters longer than any other voice actor in animation history, though they both have since been surpassed by June Foray of Rocky and Bullwinkle.
His career at Disney was the subject of the premiere episode of Disney Family Album, a 1984 series of documentaries about behind-the-scenes personalities at the studio.
Nash died on February 20, 1985 of leukemia in the Providence Saint Joseph Medical Center in Burbank, California at the age of 80 and was interred in the San Fernando Mission Cemetery in Mission Hills, Los Angeles, California. The tombstone of the grave he now shares with his wife Margaret Nash (who died in 1993) depicts a carving of Donald and Daisy Duck holding hands.
After Nash's death, Donald's voice has been taken up by Disney animator Tony Anselmo, who was trained under Nash personally. Anselmo is also among the many voiceover artists to have voiced Huey, Dewey and Louie over the years. Later characters whose voices owe considerable credit to Nash's duck voice have been voiced by actors such as Jimmy Weldon, Frank Welker, Luba Goy and Red Coffey. The most prominent of these is Weldon's Yakky Doodle for Hanna-Barbera.