McNamara, an Irish American Catholic, was born in Burbank, California. Once a professional musician, McNamara's love for music is apparent in all of his series and movies. Spending his early career in all areas of the entertainment industry, Sean climbed his way up to producing and directing countless hours of hit television for the Disney Channel. McNamara is best known for his work in the preteen film market, discovering and casting notable teen stars including Jessica Alba, Hilary Duff, Shia LaBeouf, Christy Carlson Romano, and Raven-Symoné. McNamara along with David Brookwell are the founders of the Brookwell McNamara Entertainment production company. McNamara has continued to produce and create shows for MTV's The N, Nickelodeon, Disney Channel, and most recently, Cartoon Network.
McNamara briefly appeared as the singing cowboy in the Even Stevens musical episode "Influenza: The Musical" and as an alleged alien abductee in the episode "Close Encounters of the Beans Kind". On That's So Raven, McNamara had a cameo appearance as a Plumber in the episode Out Of Control.