From 1988 through the 90s he performed at San Diego Comic-Con and other comics-related events as part of the band Seduction of the Innocent (named after the titular book by Fredric Wertham).

[4] He began his professional career at age seven and has worked on more than four hundred television episodes, eighteen films, various commercials, and scores of voice-over projects.

[6] Among Mumy's earliest television roles was six-year-old Willy in the "Donald's Friend" (1960) episode of the NBC-TV family drama series National Velvet, starring Lori Martin.

He starred in three episodes of CBS-TV's original Twilight Zone: "It's a Good Life" (S3 E8 November 1961), as six-year-old Anthony, who terrorizes his town with psychic powers (a role he later reprised along with his daughter Liliana in the "It's Still a Good Life" episode of the second revival series); "In Praise of Pip" (September 1963), as a vision of Jack Klugman's long-neglected dying son; and "Long Distance Call" (March 1961) as Billy Bayles, who talks to his dead grandmother through a toy telephone.

In 1961, Mumy was cast on CBS-TV's Alfred Hitchcock Presents series in "The Door Without a Key", featuring John Larch, who played his father in "It's a Good Life".

Mumy was cast as Mark Murdock in the "Keep an Eye on Santa Claus" (1962) episode of the ABC-TV drama series Going My Way, starring Gene Kelly.

[9] Mumy was later cast in Bless the Beasts and Children (1971) as Teft, a leader in a group of misfit teenage boys resolved to save a herd of bison from hunters.

In November 1998, he played Kellin, a Starfleet officer, in the Star Trek: Deep Space Nine episode "The Siege of AR-558", in which he assists in defeating a Jem'Hadar detachment.

Mumy's work also includes voice overs in national commercials for Bud Ice, Farmers Insurance, Ford, Blockbuster, Twix, Oscar Mayer and McDonald's.

His various musical credits include songs he has written and recorded with America, performed on tour with Shaun Cassidy, and played with Rick Springfield's band in the film Hard to Hold.

This occurred the year after the rest of the cast (including both Mumy and Harris) stated in a TV Guide article that the Sci Fi Channel planned to do a Lost in Space marathon while promoting a new film.

Harris was to appear in the planned television film Lost in Space: The Journey Home, but died before production was scheduled to start, in 2002, and it was subsequently cancelled.

In a 2010 interview on Blog Talk Radio's Lessons Learned, Rick Tocquigny was asked if Mumy was a Jonathan Harris fan before they appeared together on Lost in Space.

As a tribute to Harris, writer-director John Wardlaw added a scene that reunited Lost in Space cast members Mumy, Marta Kristen, and Angela Cartwright as the animated Ratchett family.

[21][22] Mumy and co-author Peter David published a short story, "The Black '59" (1992), in the anthology Shock Rock, edited by F. Paul Wilson.