Errol Flynn did all of his own boxing stunts in the film, and although production was shut down for a time after Flynn suffered a mild heart attack, he came back and finished the picture without ever using a double.
Soon after completing "Gentleman Jim," Flynn became embroiled in an infamous rape trial. During screenings of "Gentleman Jim," his closing line of "I'm no gentleman" was met with laughter and derision. The line was cut from the conclusion of the 1944 radio broadcast.
Arthur Shields (Father Burke) and John Loder (Carlton De Witt) fought on opposite sides of the Easter Rising of 1916: Shields fought with the Irish republicans while Loder was a second lieutenant in the British Army. Furthermore, Loder was the son of General William Lowe, the British officer to whom Pádraig Pearse surrendered on April 29, 1916. Shields had previously played Pearse in The Plough and the Stars (1936).