He is probably best known as "Will Robinson" from the 1960s television series Lost in Space and as Lennier, the alien Ambassadorial aide suffering from unrequited love in the series Babylon 5. He is well known as a player in the original Twilight Zone, especially in the episode, It's a Good Life, where he played an all powerful and amoral child who completely dominates his town.
Unlike many child actors, he entered the profession at his own insistence and his parents took pains to make sure that he matured properly in his job. This included his father, who was a successful cattle rancher, carefully investing his income and as such avoided many of the problems of former child actors like Jackie Coogan.
Mumy is also an accomplished musician who plays guitar, bass, keyboards, banjo, mandolin, harmonica and percussion. He also sings. He has released five solo CDss, Dying To Be Heard, In The Current Pandora's Box, "After Dreams Come True" and "Ghosts" as well as 9 albums with partner Robert Haimer as Barnes and Barnes. Their most famous hit is the song "Fish Heads". He is currently a member of the band Jenerators, and they have produced 2 CDs, Jenerators and Hitting the Silk.
Mumy is the co-creator with writer Peter David of the award winning children's television series Space Cases. The two have also written screenplays, comic books and short stories together.
He currently lives in Hollywood Hills, California with his wife Eileen and their two children, Seth and Liliana.