In the movie, the Gangster Squad is assembled to go after Mickey Cohen. However, in real life Chief W.H. Parker set up the squad to prevent other gangsters from taking over Cohen's rackets after Cohen went to prison on income tax evasion. Whenever the squad learned that out-of-town gangsters were coming to Los Angeles to try to set up new operations, the Gangster Squad would kidnap them, beat them up (some rumors say torture) and then send them back to wherever they came from with the warning that if they ever returned, they would be killed.
The original trailer for this movie was pulled after the Aurora, Colorado Century 16 massacre due to a shot in which four gangsters fire machine guns through a projection screen into a crowded movie theater. Warner Bros. then ordered the film back into production for a reshoot of the scene, necessitating a rescheduling of the film's release and rewrite of the script.
Ryan Gosling's exchange with Emma Stone early in the film regarding the playing of "post office" is lifted word-for-word from The Three Stooges 1934 'Three Little Pigskins' in a scene with Curly and Phyllis Crane. The short feature also starred a young platinum blonde names Lucille Ball.