Lassie began as a short story written by British author Eric Knight for the Saturday Evening Post. The story was such a success, it was expanded into a book, "Lassie Come Home". In 1943, the movie version of the book was released. The Lassie of Eric Knight's book was a tri-color collie, based on his own dog "Toots", but the dog cast for the film was a sable and white show dog from a prestigious kennel. In the famous scene where Lassie was to cross a raging river, stunt dog Pal, was called in. Pal swam across the actual raging river, nearly collapsed on the other side but under the direction of his trainer, continued to struggle to move on. Director Fred Wilcox is quoted as saying "It may have been Pal that went in the river, but it was Lassie that came out. That is my star!.” Thus began the legacy of the world's most famous line of dogs, all direct descendants of the original dog, who have played Lassie, and which continues today.

Although the fictional character of Lassie is a female, every dog that has portrayed Lassie has been male.