A Chicago native, Connolly attended the University of Illinois at Urbana-Champaign and was the Daily Illini's city editor in 1937 and 1938. From 1951 to 1966, he was a well-known gossip columnist for the Hollywood Reporter, a daily entertainment newspaper dealing with film and television productions.

Mike Connolly, columnist at The Hollywood Reporter, often reported on Hollywood's gay community in the 1950s, giving stories to the infamous scandal magazine Confidential; some believe he did this to avoid being outed himself by Confidential.

The author was also known for his 1937-38 crusade against prostitution in Champaign, Illinois, and later for his battle against communism in Hollywood. According to his biographer Val Holley, these campaigns were attempts by Connolly, who was a homosexual, to feel part of the mainstream.

He died from a kidney malfunction following open-heart surgery on November 18, 1966.