Mannix became the Vice-President of Metro-Goldwyn-Mayer. He allegedly had connections to gangs and the underworld; gossip purported that he murdered his first wife Bernice Fitzmaurice (in a make-believe high speed car crash) in 1937. He was also rumored to be complicit in cover-ups involving the deaths of film producer Paul Bern and Superman actor George Reeves, with whom his second wife Toni Mannix had a long open affair. Mannix was never formally charged as Reeves's death was ruled a suicide (as was Bern's), and no credible evidence has ever been cited to confirm the rumors. However, the 2006 film Hollywoodland (in which he was portrayed by Bob Hoskins) speculates on three theories, one of which is that one or both of the Mannixes might have caused Reeves' death.
Mannix and fellow MGM executive Howard Strickling are the subject of E. J. Fleming's book The Fixers: Eddie Mannix, Howard Strickling and the MGM Publicity Machine (2004), which also suggests that Wallace Beery, gangster Pat DiCicco, and Albert R. Broccoli beat comedian and original Three Stooges chief Ted Healy to death in the parking lot of the Trocadero nightclub in 1937, after which the studio sent Beery to Europe for several months until the heat was off. Mannix and Strickling, in their capacity as MGM's "fixers," arranged the deft cover-up, and mythical "college students" were always blamed for Healy's early death, according to the book.
Despite such unsavory rumors, Mannix was much-beloved in the film community, a gregarious and fun-loving man whose word was considered his bond by many of the stars who worked for him. Clark Gable, asked whether he had signed a new contract with MGM, famously replied, "I don't need a contract. I shook Eddie Mannix's hand."