After the Dalmatians escape from Horace and Jasper, they are lost in a snowstorm. They are found by the Collie who tells Pongo that they have shelter for them in a dairy farm. The Collie leads them to the farm where he brings food for Pongo and Perdita while the dairy cows feed the puppies. He informs them that they should head to Dinsford next where there is a Labrador whose owner is a grocer. He then goes outside to guard the door in case Horace and Jasper appear.
He is not seen for the rest of the film, but it can be assumed that he knew about the Dalmatians returning home.
The Collie apparently makes another appearance in 101 Dalmatians: The Series in the episode "Lucky to be Alone". Here, he assures Perdita that he shall keep an eye on Lucky as he travels to his Aunt Coco and Uncle Beamer Vandercream. When they arrive at the Vandercreams, he cautions Lucky to be careful.
In this episode, his human seems to be a grocer van driver, though he claims the Labrador to have the grocer human in the animated film. However, it is possible he contacted the Labrador to arrange something.