Before gaining fame in films, Moriarty worked for several years as an actor at the Guthrie Theater in Minneapolis. In 1973, Moriarty was cast as the egocentric Henry Wiggen in Bang the Drum Slowly, a film about the unlikely friendship between two baseball teammates – the second being Robert De Niro, a slow-thinking catcher who becomes terminally ill. In the same year, Moriarty starred in a TV movie adaptation of Tennessee Williams' The Glass Menagerie with Katharine Hepburn. Coincidentally, the film also featured Sam Waterston, who later replaced Moriarty as the Executive Assistant District Attorney on Law & Order. Moriarty's role in The Glass Menagerie (as "Jim," the Gentleman Caller; Waterston played the son "Tom") won him an Emmy Award for Best Supporting Actor. In 1974, Moriarty starred as rookie detective Bo Lockley in the acclaimed gritty police drama Report to the Commissioner.