Galligan was born on February 14, 1964, in New York City, the son of Carol Jean (née Wolfe), a psychologist, and Arthur John Galligan, a lawyer who was a founding partner of the law firm of Dickstein Shapiro.

He is mostly of Irish descent, although he also has some English roots and Jewish ancestry (from Austria) on his mother’s side.

[8][9][10] Prior to being cast in Gremlins, he filmed Nothing Lasts Forever, which had its theatrical release cancelled and went unreleased in any form for years.

[9] In 2016, UK progressive rock band Gandalf's Fist announced that Galligan would voice a character on their album The Clockwork Fable.

[15] Galligan married Ling-Ling Hu Ingerick at the Yale Club in New York on September 25, 2005; they divorced in 2010.