Galligan's most notable role was as Billy Peltzer in the Michael Finnell-produced 1984 film Gremlins. Following the first Gremlins film, Galligan appeared in Nothing Lasts Forever. In 1988, he starred as Mark Loftmore in the horror comedy Waxwork. He reprised his role as Billy Peltzer in the 1990 sequel Gremlins 2: The New Batch. In 2016, UK progressive rock band Gandalf's Fist announced that Galligan would voice a character on their album The Clockwork Fable.