When filming was completed, the footage ran to a total of eight to ten hours. Director Sergio Leone and editor Nino Baragli trimmed the footage to around six hours, with the plan of releasing the film as two three-hour movies. The producers refused this idea, and Leone had to further cut the film down to three hours and forty-nine minutes.
A few days before the film's premiere in 1984, Treat Williams found out the two-hour version, not the three hour and forty-nine minute version, would be shown in theaters. He was heard to have said that no one would understand the movie in the shortened version. Indeed, the film did not do well at theaters, and was shut out of the Oscars, and received no nominations. When the video cassette and DVD versions were released in the original three hour and forty-nine minute version, the film ultimately found commercial and critical success.
Robert De Niro requested a private meeting with renowned crime boss Meyer Lansky to prepare for his role as David "Noodles" Aaronson. The request was denied.
Joe Pesci originally auditioned for Max, but Sergio Leone convinced him that he wouldn't be quite right for the role. As a favor to Pesci's friend Robert De Niro, Leone told Pesci that he could pick whichever of the available roles he wanted as his own instead. He chose the part of Frankie, which was considerably larger in the original script than it is in the finished movie.
This was Jennifer Connelly's first feature film role. According to her during an interview, she was 11 during principal photography.