Toy Story 2 was the first Pixar feature film to receive a sequel, arriving before Cars 2.
Toy Story 2 was released in 1999, three years after the original Toy Story. It began as a project intended for a smaller release but was expanded into a major theatrical sequel. The film follows Woody after he is stolen by a toy collector and learns about his past as a television character.
Cars premiered in 2006, and its sequel, Cars 2, followed in 2011. That means the Cars films belong to a later wave of Pixar follow-ups. Toy Story 2 established that Pixar could continue a successful feature story while expanding its characters and world.
Finding Dory and Incredibles 2 were also later Pixar sequels, but neither preceded Toy Story 2. The question concerns the first sequel among Pixar feature films, not the first film in a franchise to be announced or planned.