The songs sung by the male voices are all authentic Welsh. The song sung at the opening is "Men of Harlech".
The title of the novel appears in two sentences. It is first used in Chapter XXX, after the narrator has just had his first sexual experience. He sits up to "look down in the valley." He then reflects: "How green was my Valley that day, too, green and bright in the sun." Needless to say, this sexual experience was cut out of the film. The phrase is used again in the novel's last sentence: "How green was my Valley then, and the Valley of them that have gone."
This film was selected to the National Film Registry, Library of Congress, in 1990.
Two major factors entered into the decision to shoot the film in Southern California: (1) the continuous bombing of Britain by the Nazis; (2) the nervousness of Fox executives about the film's pro-union storyline. These factors, and William Wyler's reputation for perfectionism, swayed Fox to keep the filming done in the U.S.
When the movie was intended to be a four-hour epic Tyrone Power was going to play Huw as an adult.