John was forced to use his middle initial "M." for his professional name because there was already a "John Jackson" registered with the Screen Actors Guild when he joined the union and SAG rules prohibit two or more members from using exactly the same name.
Jackson started acting in his late 20s, at a community theater. In 1980, Jackson went to New York City, and later to Hollywood, to further his acting career. He had small roles in a number of television and motion picture productions, most notably a recurring role in the series Wiseguy. In 1986, he appeared in the TV film Blind Justice, starring Tim Matheson. In 1987, Jackson had a small role in the Macgyver episode "Birth Day" as LA police sergeant Meechum who recognises MacGyver's flag help signal, saying he does so because he spent four years in the Navy. In 1992, Jackson played Captain West, a JAG Officer, in the American drama film A Few Good Men.