Following his graduation from high school, Moore, as an 18-year-old member of the Flint school board, began his populist assault on what he viewed as the injustices of American capitalism. In 1976, after having attended but not graduated from the University of Michigan at Flint, Moore started a radical weekly newspaper, the Flint Voice (later Michigan Voice), which he edited for 10 years. He was later hired to edit the San Francisco-based left-wing magazine Mother Jones but was fired after a few months (he later accepted an out-of-court settlement for a wrongful-dismissal suit).

Returning to Flint, Moore filmed his first documentary, Roger & Me (1989), which chronicles the effects of unemployment in Flint due to the closing of two General Motors (GM) factories and the company's longer-term policy of downsizing. At the centre of the film were Moore's “in-your-face” efforts to gain an audience with GM's chairman, Roger Smith. Mixing humour and poignancy with indignation, Roger & Me was a hit with critics and at the box office. Moore subsequently moved to New York City and established Dog Eat Dog Films. He also created an organization to finance social-action groups and other filmmakers.

After producing three television series and other limited-release films—including the comedy Canadian Bacon (1995), in which a U.S. president starts a cold war with Canada in order to boost his approval ratings—Moore achieved major success with Bowling for Columbine (2002). The film, which profiles gun violence in the United States, won the Academy Award for best documentary. In his next documentary, Fahrenheit 9/11 (2004), Moore criticized U.S. Pres. George W. Bush (Bush, George W.)'s handling of the September 11 attacks and the administration's decision to start the Iraq War. Although highly controversial, it won the Golden Palm at the Cannes film festival and earned more than $222 million worldwide to become the highest-grossing documentary. In 2007 Moore released Sicko, an examination of the health care industry in the United States.

In addition to filmmaking, Moore wrote a series of best-selling books, including Downsize This! (1996); Stupid White Men (2002), which assails the legitimacy, methods, and motives of President Bush's administration; and Dude, Where's My Country? (2003), a call for “regime change” in the United States. He later wrote Mike's Election Guide (2008), a guidebook to the 2008 U.S. presidential election.