"Someone to Watch Over Me," written by George Gershwin and Ira Gershwin, was used as the main theme song. Warner Bros. had acquired the rights to the entire Gershwin catalog in 1944 in preparation for their biopic Rhapsody in Blue (1945).
The stage play by Norman Krasna was produced by Richard Rodgers and Oscar Hammerstein II in association with Joshua Logan, opening at the Booth Theatre on February 4, 1947 and transferring to the Music Box Theatre on March 17 of that year, where it closed on February 7, 1948 with a total of 423 performances. The Broadway cast included William Prince as John Lawrence, Nina Foch as Mary McKinley, Harry Bannister as Harwood Biddle, Lyle Bettger as Lt. Victor O'Leary, Ralph W. Chambers as Oscar Dugan, Tom Ewell as Fred Taylor, Ann Mason as Phyllis McKinley, Max Showalter as George Beechwood, and Loring Smith as Senator James McKinley.
The casting of Patricia Neal was something of an upset at the time. Nina Foch had skyrocketed to fame when she created the role on stage, following four years spent toiling as a B-player at Columbia Pictures. When she finally made a name for herself on Broadway, it was assumed she would reprise her role on screen. However, in the same theatrical season as "John Loves Mary" premiered (1946-47), Patricia Neal made an auspicious stage debut in Lillian Hellman's "Another Part of the Forest," winning the very first Tony Award for Best Featured Actress in a Play. Neal was quickly signed to a film contract by Warner Bros., which had acquired the rights to "John Loves Mary," and it was announced that she would make her screen debut in the film version. Neal scored big at Warners, particularly when she began a notorious, well-publicized affair with Gary Cooper during the filming of The Fountainhead (1949), and she was a leading lady from that point on. In her memoirs, Foch revealed that her career never recovered from having lost John Loves Mary (1949), as she continued to play supporting roles at various studios, the two highlights of her screen canon being her touching performances in An American in Paris (1951) and Executive Suite (1954).