Plans to film in Wales were abandoned due to WWII; an 80-acre set was built in the Santa Monica Mountains at Brent's Crags, near Malibu. The design of the village was based on the real Cerrig Ceinnen and nearby Clyddach-cum Tawe in Wales.
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.
For the scene where the miners greet their women by putting their earnings in baskets, actress Maureen O'Hara stopped the scene's filming once she noticed that her basket was a modern Kraft basket and not a basket of the movie's period. Director John Ford was so upset by being corrected in front of the cast and crew that he closed down the set and told O'Hara to wait on a nearby hill until he called for her. Fuming, O'Hara waited an hour before an assistant came to retrieve her but was satisfied to see that the basket had been changed upon her return.
The poster from the mine managers telling the miners that their wages were being cut mistakenly uses the American spelling of "labor". It would never be spelled that way in rural Wales; instead it would have used the British version "labour".
The author of the novel, Richard Llewellyn had claimed to have based the book on his own knowledge of the Gilfach Goch area, but this was proven false, as Llewellyn was English-born and spent little time in Wales. As it turned out, he had actually gathered his facts from conversations with local mining families.
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."
The author continued the story about Huw Morgan's life in 3 sequels. 'Up into the Singing Mountain' (1960) in which Huw emigrates to Argentina; 'Down Where the Moon is Small' (1966), Huw's life in Welsh-speaking parts of Argentina; and 'Green, Green My Valley Now' (1975), in which Huw returns to Wales. None of these have been made into films, and 'How Green Was My Valley' is still the most consistently popular novel of the series.
As well as this film, the book has twice been adapted by the BBC as a serial for television, in 1960 and 1975. The 1975 production - scripted by Elaine Morgan - starred Stanley Baker, Siân Phillips, and Nerys Hughes. It was also adapted as a Broadway musical, called A Time for Singing, which opened at the Broadway Theatre, New York, on May 21, 1966. The music was by John Morris; book and lyrics were by Gerald Freedman and John Morris. The production was directed by Mr. Freedman, and it starred Ivor Emmanuel, Tessie O'Shea, Shani Wallis, and Laurence Naismith.
John Ford became the first director to win three Academy Awards for Best Director, having previously won for The Informer (1935) and _The Grapes of Wrath_. 11 years later he would win his fourth for The Quiet Man (1952), a record unmatched by any other director.
Sara Allgood clashed repeatedly with John Ford during the making of the film, something the director found rather unsettling as he wasn't used to having his authority questioned. Nevertheless her performance was recognized by the Academy with a Best Supporting Actress nomination.
Roddy McDowall had only been in America for two weeks before being cast in the leading role of Huw. He had been evacuated from Great Britain with his mother and sister to keep out of harm's way of Hitler's bombardment of the country.