Paul Gallico, the author of the movie's source novel, was invited to the set during filming, and quickly came to dislike "the great god Disney". Therefore, he was vastly amused when one of the felines portraying Thomasina held up filming for two days by flatly refusing to perform a stunt for which she had been trained, in spite of Walt Disney's frustrated bellowing. Paul Gallico recalled in his memoirs: "I was proud of that cat!" As with many films with animals, more than one cat was used to portray Thomasina. Disney introduced the film when it aired on television (it's included in the DVD special features), during which he indicated that Paul Gallico was his friend.
The film was first broadcast on television in three one-hour installments over three weeks on Le monde merveilleux de Disney (1954).
As with most animals in films, several cats were used. Unfortunately, the cats were not a good match for each other: while all are red tabby cats, one of them is a "classic" tabby with markings that show a "bullseye" on its side, and another is a "mackerel" tabby with stripes on the side.
During a scene in which Dr. McDhui is performing surgery, he asks his assistant for the "Spencer Wells". This is a type of forceps, named for Sir Thomas Spencer Wells, physician to Queen Victoria and a pioneer in abdominal surgery.
When Karen Dotrice first appears in the film, before the opening titles, and says, "Thomasina!", her voice is actually that of Robie Lester, who also sings the theme song.