Author of the book "Thomasina" Paul Gallico was present on the set during filming by special invitation. As Gallico quickly came to dislike "the great god Disney", he was vastly amused when one of the felines portraying Thomasina held up filming for two days when she flatly refused 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 put a foreword on the film (DVD special features) and indicated 'Paul Gallico' was his friend.
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.
In the scene at the circus where Lori goes to check on the abused animals, there is a man riding a horse in a ring. At first the horse is white. Later the same horse is gray spotted.
Mary's father says that the cat Thomasina must be put to death and her body disposed of because she has tetanus and could spread the disease to other animals and people. Yet Mary and the village children later are allowed to use Thomasina's corpse as the centerpiece for the cat's funeral.