The film's opening action sequence was shot by a helicopter camera, placed on a gyro-stabilized mount. At the time of this production, aerial helicopter photography was not common. Although helicopters had been used in filming by 1947, mainly for aerial views or landscapes, this is one of the first times an action scene was filmed from the sky. The most difficult thing about it was to keep the actors in focus. It took four takes; they eventually chose the second one.
Robert Mitchum lobbied unsuccessfully for the role of Chicamaw. He told Nicholas Ray that he was very familiar with bank robbers and chain gangs, and even cut his hair and dyed his hair black (in the original treatment Chicamaw was an Indian). He was rejected because he had recently been nominated for an Oscar, and a supporting role was considered unworthy for a rising star.
According to Eddie Muller on TCM's Noir Alley (2017-10-15), Farley Granger and Cathy O'Donnell first appeared together in They Live by Night (1948), filmed at RKO Pictures in 1947 but put on the shelf by new studio boss Howard Hughes. Former RKO Production Chief Dore Schary feared the picture would never be released, so he paired the actors again in Side Street (1949) at his new studio, M-G-M. Hughes learned of this and, wanting to beat Schary to the punch, finally released "They Live by Night" in the USA in November 1949, just a few weeks before "Side Street" opened nationwide.