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". The same year, Mumy starred as little Jackie in the episode "Bang! You're Dead", featuring
Marta Kristen, who later played his sister Judy on
Lost in Space. 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. His fellow guest stars were
Cloris Leachman (who played his mother in "It's a Good Life"),
Steve Brodie, and
Frank McHugh. At age eight, Mumy appeared in
Jack Palance's ABC-TV
circus drama
The Greatest Show on Earth (1963); he was cast as Miles, a parentless boy, in the
Perry Mason episode "The Case of the Shifty Shoebox" (1963), and he portrayed Freddy in the "End of an Image" (1963) episode of NBC-TV's modern Western series
Empire, starring
Richard Egan. In 1964, he was cast as
Richard Kimble's nephew in ABC-TV's
The Fugitive episode, "Home Is the Hunted"; as Barry in the
NBC-TV medical drama
The Eleventh Hour episode "Sunday Father"; as himself three times in the ABC sitcom
The Adventures of Ozzie and Harriet; in the
Disney film
For the Love of Willadena; and as a troubled orphan taken in by the Stephenses in the
Bewitched fantasy sitcom episode "
A Vision of Sugarplums" (December 1964), on ABC-TV. Mumy was reportedly the first choice to portray
Eddie Munster in the 1964 CBS situation comedy
The Munsters, but his parents objected to the extensive makeup requirements. The role instead went to
Butch Patrick. Mumy appeared in one episode as a friend of Eddie. Mumy guest starred in an episode of NBC-TV's
I Dream of Jeannie, "Whatever Became of Baby Custer?" (1965). That same year, he also appeared in an episode of
Bewitched titled "Junior Executive" (1965), in which he played a young Darrin Stephens. in
Dear Brigitte, 1965 Mumy starred in
Dear Brigitte (1965), a film adaptation of the novel
Erasmus with Freckles, as Erasmus Leaf, a child mathematical genius who develops a crush on
Brigitte Bardot (played by herself in the film). His parents, played by
James Stewart and
Glynis Johns, attempt to manage his obsession.
Lost in Space and beyond From 1965 to 1968, Mumy portrayed Will Robinson in
Lost in Space, the recipient of numerous warnings (including "Danger, Will Robinson") from the show's
robot character, voiced by
Dick Tufeld. 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. He also played a musician friend of
Cliff DeYoung's character in the television film
Sunshine (1973), later reprising the role in
Sunshine Christmas (1977) and in the TV series
Sunshine (1975). In 1974, Mumy played Nick Butler in the pilot episode of NBC's
The Rockford Files and made an appearance in a later episode in season 1 as a sidewalk artist. In 1988, he played Ben Matlock's genius nephew, Dr. Irwin Bruckner, on
Matlock. In 1996, Mumy was a writer and co-creator of
Space Cases, a
Nickelodeon television show with themes similar to those of
Lost in Space. Between 1994 and 1998, he played the ambassadorial aide
Lennier in the syndicated science fiction series
Babylon 5. 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. To Mumy's delight, his character was human this time due to makeup time and his distaste as being known as an "alien actor"; while playing
Lennier in
Babylon 5, he was required to wear prosthetic makeup. Mumy later appeared in a 2006 episode of
Crossing Jordan and in the
Syfy original film
A.I. Assault. In 2018, Mumy appeared in the pilot episode of the
Netflix remake series
Lost in Space. His character's name is Dr. Z. Smith, in homage to the character played by
Jonathan Harris in the 1965 television series. ==Voice acting career==