Ullman is best known for his starring role as Phil Diffy, a teen from the year 2121 on Disney Channel's "Phil of the Future", a series that debuted June 2004. He also starred as Roscoe in Disney's made-for-television film "Pixel Perfect." During this period, Ullman appeared in other Disney Channel programming, such as a guest role on "That's So Raven", and was an identified member of the Disney Channel Circle of Stars, appearing with other members of the Circle in the music video for "A Dream is a Wish Your Heart Makes," where he sang as well as played the drums.