(written from a Production point of view)
Charles William "Bill" Mumy Jr. (born 1 February 1954; age 67), sometimes credited as Billy Mumy, is an actor from San Gabriel, California, who played Crewman Kellin in the Star Trek: Deep Space Nine seventh season episode "The Siege of AR-558". A friend of Ira Steven Behr, Mumy had wanted to appear on the show for some time prior to this episode but he only wanted to play a Human, as he had worn alien makeup for years as a cast member of Babylon 5. (Star Trek: Deep Space Nine Companion (p. 626))
Mumy's photographs from DS9 can be found here on his website. 
Before his appearance on Deep Space Nine, Mumy had worked in Star Trek on one previous occasion: he co-wrote (with Peter David) three issues of the DC TOS volume 2 series of comics. The story depicted the USS Enterprise-A's encounter with The Worthy, a group of alien characters modeled after the cast of Lost in Space (in which Mumy had played a primary character), who had, predictably enough, been lost in space for centuries.
Bill Mumy is perhaps best known for his childhood role as young Will Robinson on the classic science fiction series Lost in Space (1965-1968), the show broadcaster CBS Broadcasting had in development at the time when they turned down Gene Roddenberry's April 1964 Star Trek is... pitch. Other Star Trek alumni he worked with during the show – which became a contemporary franchise contender for Star Trek: The Original Series, much like Deep Space Nine and Babylon 5 would become in their day – are Keith Taylor, Michael J. Pollard, Michael Ansara, Torin Thatcher, Liam Sullivan, Sherry Jackson, Peter Brocco, Ted Cassidy, Malachi Throne, John Abbott, Ron Gans, Arthur Batanides, Bart La Rue, Lou Wagner, Don Eitner, John Crawford, Byron Morrow, Abraham Sofaer, Stanley Adams, writers Shimon Wincelberg, Carey Wilber and Robert Hamner, directors Alexander Singer, Tony Leader, and Leo Penn, and composers Alexander Courage, Gerald Fried, and Fred Steiner.
By the time he took on that role, however, he had already been acting in television and films for five years, having joined show business at his own insistence – at the age of five. Prior to Lost in Space, Mumy appeared on such classic shows as Alfred Hitchcock Presents, Wagon Train, Perry Mason, The Fugitive (in an episode with Andrew Prine, Clint Howard, and James B. Sikking), The Munsters, Bewitched, and three episodes of The Twilight Zone, most notably 1961's "It's a Good Life" (which co-starred Don Keefer and was written by Jerome Bixby). In addition, he made appearances in films like 1963's A Child Is Waiting (co-starring Lawrence Tierney) and Palm Springs Weekend and 1965's Dear Brigitte.
Following the cancellation of Lost in Space in 1968, Mumy starred in the 1969 Walt Disney film Rascal, which also featured Star Trek: The Original Series guest actor John Fiedler. In 1971, he starred in Stanley Kramer's drama Bless the Beasts & Children, and in 1973, he had a supporting role in the classic biographical film Papillon. This latter film co-starred fellow Star Trek alumni Peter Brocco, Gregory Sierra, William Smithers, Ron Soble, Vic Tayback, and Anthony Zerbe.
Also in 1973, Mumy portrayed the role of Weaver in the TV movie Sunshine, starring Cliff DeYoung and Meg Foster, also featuring Noble Willingham, and directed by Joseph Sargent. Mumy, Foster, and De Young would go on to reprise their roles in a short-lived 1975 series also entitled Sunshine and a 1977 TV movie follow-up, Sunshine Christmas. Besides these, Mumy's only other TV credits during the 1970s were two episodes of The Rockford Files, including the pilot.
Mumy did very little in film or television during the 1980s. His only notable credits during this time was an appearance in Twilight Zone: The Movie (in a segment remaking the aforementioned "It's a Good Life" episode, with Mumy in a different role, and co-starring Dick Miller and William Schallert, while John Larroquette, Peter Brocco and Bill Quinn also appeared in the movie in other segments) and a guest spot on Matlock (in an episode with Michael Durrell, Lawrence Pressman, and Carolyn Seymour). His career picked up some speed in the early 1990s with roles in the films Captain America (1990, which also featured Ronny Cox and Michael Nouri) and Double Trouble (1992, co-starring James Doohan) and appearances on The Flash and Superboy.
In 1994, Mumy began starring in Babylon 5, playing the regular character Lennier. Other Bablyon 5 regulars and semi-regulars to have appeared on Star Trek include Walter Koenig, Andreas Katsulas, Caitlin Brown, Mary Kay Adams, Robert Rusler, Patricia Tallman, Tracy Scoggins, John Vickery, Marshall R. Teague, John Schuck, Robin Sachs, Rick Scarry, Anthony De Longis, Patrick Kilpatrick, and Reiner Schöne.
In 1996, he and Peter David co-created the short lived science fiction series Space Cases for the children's cable channel Nickelodeon. George Takei appeared in several episodes as a recurring villain.
After Babylon 5 ended in 1998, and after he appeared on Deep Space Nine, Mumy went on to reprise his role as the supernatural Anthony Fremont from the "It's a Good Life" episode of The Twilight Zone in a follow-up story for UPN's 2002-03 version of The Twilight Zone entitled "It's Still a Good Life", written by Ira Steven Behr and directed by Allan Kroeker. In 2006, he co-starred with Trek regulars Michael Dorn, Robert Picardo, and George Takei in the TV movie Shockwave (aka A.I. Assault). Additionally, Mumy continues to expand his credits in the field of voice acting, having lent his voice to a number of animated TV programs and direct-to-video films, including an episode of Batman: The Animated Series, co-starring Loren Lester.
Mumy is also a musician and is one-half of the popular "novelty rock" duo Barnes & Barnes along with childhood friend Robert Haimer. The duo is perhaps most famous for their song "Fish Heads". He and Haimer co-wrote and starred in an animated short based on this song in 1982.
Mumy has two children - son Seth and daughter Liliana - both of whom are actors. Seth appeared with his father in the movie Three Wishes; Liliana co-starred as the all-but-omnipotent offspring of her father's Twilight Zone character Anthony Fremont in "It's Still a Good Life".
- DC TOS volume 2 (cowritten)