Andrew Jackson Milligan Jr. (February 12, 1929 – June 3, 1991) was an American playwright, screenwriter, actor, and filmmaker, whose work includes 27 movies made between 1965 and 1988. Following a brief acting career, Milligan worked as a stage director in the off-off-Broadway milieu. He then turned to filmmaking, starting with an arthouse short but quickly moving to exploitation films. His works, all of them low-budget, include a number of horror films and are known for their violence, misanthropy and misogyny.

Andrew Jackson Milligan Jr. was born on February 12, 1929, in St. Paul, Minnesota. He was a self-taught filmmaker and was responsible for much of the creative activity on his movies (including cinematography and costume design).

Milligan was an "army brat"; his father, Andrew Milligan Sr. (b. February 17, 1894, d. October 25, 1985), was an officer in the U.S. Army who served in the military for over 50 years (retiring in the mid-1960s holding the rank of captain). The family frequently moved around the country as a result of this. Milligan's mother, Marie Gladys Hull (b. June 10, 1900, d. November 12, 1953), was an overweight alcoholic with severe physical and mental health problems who served as an inspiration for some of Milligan's movie characters. Milligan's parents met and married in 1926. He was close to his father, who affectionately called him 'Junior', but had a troubled relationship with his mother who, according to family members, was both physically and mentally abusive towards all her children as well as her husband.

Milligan had an older half-brother named Harley LeRoy Hull (b. July 10, 1924, d. February 17, 1998) and a younger sister named Louise Milligan Howe (b. May 27, 1931, d. January 2, 2021).

After finishing high school in 1947, Milligan enlisted in the U.S. Navy, serving for four years. After his honorable discharge in 1951 he settled in New York City, where he acted on stage, and opened a dress shop.

During the late 1950s, Milligan became involved in the nascent off-off-Broadway theater movement where he mounted productions of plays by Lord Dunsany and Jean Genet at the Caffe Cino, a small Greenwich Village coffeehouse that served as a hothouse for rising theater talent like Lanford Wilson, Tom Eyen and John Guare. Milligan also became involved with directing low-key theater productions at the La MaMa Experimental Theatre Club. During this period, he operated and designed for a clothing boutique named Ad Lib and used his dressmaking skills to costume many theatrical productions.

In the early 1960s, Milligan began making movies. He met some of the actors for his early movies at Caffe Cino. His first released movie was a 30-minute black-and-white 16 mm short drama titled Vapors (1965). The movie, set on one Friday evening in the St. Mark's Baths, a gay bathhouse for men, portrays an emotionally awkward and unconsummated meeting between two strangers. Milligan was later employed by producers of exploitation films, particularly William Mishkin, to direct softcore sexploitation and horror features, many featuring actors known from the off-off Broadway theater community.

Most of his early exploitation movie fell into the genre of morality play. Milligan's plays and movies explored topics of transgression and punishment, dysfunctional family relationships, repressed sexuality, homosexuality and physical deformity, and include such titles as Depraved! (1967), The Naked Witch (1967), The Promiscuous Sex (1967), The Degenerates (1967), The Filthy Five (1969), Gutter Trash (1969), The Ghastly Ones (1968), Seeds of Sin (1968), Fleshpot on 42nd Street (1973), The Rats Are Coming! The Werewolves Are Here! (1973), and Guru, the Mad Monk (1970). Most of Milligan's early works are currently considered lost films.