Charles William Mumy Jr. (/ˈmuːmi/; born February 1, 1954) is an American actor, writer, producer, and musician. He came to prominence in the 1960s as a child actor whose work included television appearances on Bewitched, I Dream of Jeannie, The Twilight Zone, Alfred Hitchcock Presents, and a role in the film Dear Brigitte, followed by a three-season role as Will Robinson in the 1960s sci-fi series Lost in Space. Mumy later appeared as lonely teenager Sterling North in the film Rascal (1969) and Teft in the film Bless the Beasts and Children (1971).

In the 1990s, Mumy performed the role of Lennier in all five seasons of the sci-fi TV series Babylon 5 and narrated the Emmy Award–winning series Biography.

Mumy is also a guitarist, singer, songwriter, and composer. He is an Emmy nominee for original music in Adventures in Wonderland (1992). As a musician, Mumy performs as a solo artist, an occasional guest performer, and formerly as half of the duo Barnes & Barnes before bandmate Robert Haimer died in 2023. From 1988 through the 1990s 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). The band released one CD, The Golden Age.

Mumy was born in San Gabriel, California, to Charles William Mumy, a cattle rancher, and Muriel Gertrude Mumy (née Gould). 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 voiceover projects. He has also worked as a musician, songwriter, recording artist, and writer.

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.