Voice cast
Phil Harris as J. Thomas O'Malley (full name: Abraham de Lacy Giuseppe Casey Thomas O'Malley) – A feral cat who befriends Duchess and her kittens, becoming a father figure and a step-father to the kittens and falling in love with Duchess. For cultural reasons, the Italian dubbing of the film changes him to Romeo, er mejo der Colosseo ("Romeo, the best [cat] of the Colosseum" in Romanesco dialect), an Italian cat from Rome speaking with a strong Roman accent; the reason for this change is that alley cats were well known for frequenting the Colosseum at the time.
Eva Gabor as Duchess – Madame Adelaide's refined and elegant Turkish Angora cat and mother of the three kittens, who believes she is forced to choose between loyalty to Madame and her own attachment to Thomas O'Malley at the end of the film. Robie Lester provided the singing voice for Duchess.
Gary Dubin as Toulouse – The oldest kitten, who idolizes all alley cats, especially O'Malley. He wants to appear tough, but is actually fairly laidback and easygoing. He is also a talented painter, loosely based on French painter Henri de Toulouse-Lautrec.
Liz English as Marie – The middle kitten and the only girl. She is a dreamer. She comes off as spoiled, often imperious, sassy or snobbish towards her brothers, but does her best to be a "lady" like her mother. She is an accomplished singer. She is named after the queen of France Marie Antoinette.
Dean Clark as Berlioz – The youngest kitten. He may seem quiet, but is quite mischievous. He is sweet-natured, but easily annoyed and hard to impress, often the first to make a snide remark. He is a talented pianist, named after the French composer Hector Berlioz.
Sterling Holloway as Roquefort – A house mouse and a friend of the cats who assists in the expulsion of Edgar.
Scatman Crothers as Scat Cat – O'Malley's best friend and leader of a gang of jazz-playing alley cats. Scat Cat plays the trumpet.
Paul Winchell as Shun Gon – A Chinese cat in Scat Cat's gang. He plays both the piano and drums made from pots.
Lord Tim Hudson as Hit Cat – An English cat in Scat Cat's gang. He plays acoustic guitar.
Vito Scotti as Peppo – An Italian cat in Scat Cat's gang. He plays the concertina.
Thurl Ravenscroft as Billy Boss – A Russian cat in Scat Cat's gang. He plays the double bass.
Nancy Kulp as Frou-Frou – Madame Adelaide's Palomino carriage horse and Roquefort's companion, who subdues Edgar. Ruth Buzzi provided her singing voice.
Pat Buttram as Napoleon – A bloodhound who attacks Edgar when he intrudes on the farm where he lives. Whenever his cohort Lafayette makes a suggestion, Napoleon insists that he is in charge, then adopts Lafayette's suggestion as his own.
George Lindsey as Lafayette – A Basset Hound and Napoleon's companion. He sometimes proves smarter than Napoleon but is also more timid.
Hermione Baddeley as Madame Adelaide Bonfamille – A wealthy former opera singer and the owner of Duchess and her kittens.
Charles Lane as Georges Hautecourt – Adelaide's eccentric lawyer, who is also her oldest friend. He is extremely lively, despite his advanced age.
Roddy Maude-Roxby as Edgar Balthazar – Adelaide's greedy but dim-witted butler, who tries to get rid of her cats in order to inherit her fortune.
Monica Evans as Abigail Gabble – Amelia's twin sister, a goose who befriends the cats.
Carole Shelley as Amelia Gabble – Abigail's twin sister, a goose who befriends the cats.
Bill Thompson as Uncle Waldo – The drunken gander uncle of Abigail and Amelia. This was Thompson's final film role.
Peter Renaday as French Milkman, Le Petit Cafe Cook, and Truck Movers (uncredited)
