Rosen began his career as host, writer, and producer of the Los Angeles children's TV series Dusty's Treehouse, which ran locally and later in syndication from 1968 until 1980. Moving to the world of animation, Rosen supplied character voices on Kissyfur (as Floyd and Stuckey), My Little Pony and Friends, Ironman, Biker Mice from Mars, and others. He served as voice director on several of the above, and other Marvel Productions series, as well as the video game Gabriel Knight: Sins of the Fathers (with the voice of Tim Curry). Most recently, in 2005, Rosen appeared on-camera in two episodes of the HBO series Rome.