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.
The film was originally slated for an early September, 2012 release date but ended up getting bumped until January of 2013 so some additional scenes could be filmed.
Ryan Gosling's character makes a joke where he pretends that the only famous Mickey he has heard of is Mickey Mouse. As a child, Ryan Gosling starred in The All New Mickey Mouse Club.
Officer Darryl Gates (Josh Pence), Chief Parker's driver, went on to become the Chief of the LAPD himself. His headquarters, Parker Center, was named for his former boss.
In the movie, the Gangster Squad is assembled to go after Mickey Cohen. However in real life Chief 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 an out-of-town gangster was coming to Los Angeles to try and 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 the gangster ever returned, he would be killed.