André Previn, who provided the unseen piano solos for the film, received his first onscreen credit for It Happened in Brooklyn (1947). Previn, who was only 17 at the time of production, had been a member of the M-G-M music department for several years prior to his work on this film. Previn went on to work as both a composer and conductor for many films and won a number of Academy® Awards before becoming principal conductor of the London Symphony and other internationally known orchestras.
Three songs from It Happened in Brooklyn (1947) made it onto the music charts in 1947/48. They were "I Believe," "It's The Same Old Dream" and "Time After Time."
In this film, Frank Sinatra introduced the now standard "Time After Time," which charted at #17 in 1947. It was later re recorded, by Frank, in 1959 as the B side to "French Foreign Legion." In 1960, Frankie Ford's rendition of the song charted at #75 US. Interestingly, that version fared much better than all, in Brooklyn, reaching NYC's Top 10. In 1966, Chris Montez' version peaked at #36 US.
The unit patch on Danny's uniform jacket is that of the 84th Infantry Division, formerly known as the 84th Division (Institutional Training) and nicknamed "The Railsplitters." The nickname derives from the divisional tradition that the unit traces its lineage back to the Illinois militia company in which Abraham Lincoln (known as "the Railsplitter") served.
Peter Lawford wasn't really a singer, but he did sing in at least three MGM musicals: It Happened in Brooklyn (1947), Good News (1947), and Osterspaziergang (1948).