John Ford became the second director to win three Academy Awards for Best Director, having previously won for The Informer (1935) and The Grapes of Wrath (1940). 11 years later, he would win his fourth for The Quiet Man (1952) (which also starred Maureen O'Hara), a record unmatched by any other director.

When the miners greet their women by putting their earnings in baskets, 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. 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.

John Ford often referred to this film as his favorite.

"How Green Was My Valley" famously beat Orson Welles' Citizen Kane (1941) to a Best Picture Oscar.

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 Nazi bombardments of the islands.

John Ford referred to Philip Dunne's script as "nearly perfect a script as could be possible".

Historians have called the way the wind plays with O'Hara's veil when she leaves the church after her wedding a stroke of luck for John Ford. Far from it, he had instructed the crew to set up wind machines to fan the veil into a perfect circle behind her head then blow it straight up into the air.

Sara Allgood was the only actor who gave John Ford any trouble. At one point, she complained that a scene they were about to shoot wouldn't play. Ford called writer Philip Dunne to the set and relayed her opinion to him. Having worked with Ford before, Dunne knew what to do. He ripped the scene out of the script and said, "Now it plays!" Then Ford turned to Allgood and said, "The sonofabitching writer won't do anything to help us, so we'll have to shoot it the way he wrote it."

The film was shot in black and white because the color of flowers in Southern California did not match those found in Wales.

Donald Crisp and Sara Allgood were always first choice to play the father and mother.

150 builders took six months to construct Richard Day's elaborate set design.

As was John Ford's practice, he cut the film in the camera. Almost every shot he took wound up in the edited film. He also rarely shot more than three takes of any scene.

The songs sung by the male voices are all authentic Welsh. The song sung at the opening is "Men of Harlech" (it was also used as the rallying cry for the Welsh soldiers defending against a Zulu attack in Zulu (1964)).

Anna Lee discovered she was pregnant halfway through filming and made a point of not telling John Ford about her condition for fear of losing her role.

Darryl F. Zanuck originally intended the film to be a four-hour epic to rival Gone with the Wind (1939).

William Wyler was all set to direct on location in Wales, and Laurence Olivier, Katharine Hepburn and Tyrone Power were all being courted for parts in the film.

Maureen O'Hara (Angharad Morgan) was the film's last surviving cast member when she died on October 24, 2015 at the age of 95.

Maureen O'Hara was 19 years old at the time of filming.

Darryl F. Zanuck paid $300,000 for the rights to the novel.

Two of Huw's sisters from the book - Ceridwen (older) and Olwen (the youngest sibling) - do not appear in the film.

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.

With 10 Academy Award nominations, this was the most nominated film of its year.

The biggest grossing film of 1941.

The author of the novel, Richard Llewellyn, had claimed to have based the book on his own knowledge of the Gilfach Goch area of Wales, 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.

Principal photography took only two months.

William Wyler left the production to make The Little Foxes (1941) instead.

Roddy McDowall's screen test, which won him a Fox contract, was directed by William Wyler. Dr. Gruffydd was played by Alexander Knox in the test.

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.

When the movie was intended to be a four-hour epic, Tyrone Power was going to play Huw as an adult.

The mining village set cost $110,000 to construct and was modelled on the towns of Cerrig Ceinnen and Clydach, Cwmtawe. The studio brought in blocks of coal weighing over a ton apiece for the construction of the mines. To create the impression that coal slag covered the landscape in the opening and closing scenes, John Ford had the hillside painted black.

Arthur Shields (Mr. Parry) and John Loder (Ianto Morgan) 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).

Roddy McDowall is credited in the opening credits as Master Roddy McDowall, but the "Master" is omitted in the end credits.

For the most part, the atmosphere on the set was totally congenial, with John Ford trusting his cast to deliver strong performances with a minimum of guidance. The one problem for Maureen O'Hara occurred when she pointed out - in front of the cast and crew - that the basket with which she and Sara Allgood were supposed to receive Donald Crisp's weekly wages was not of the period. In response to what he considered her breach of etiquette, he removed her from the scene. An hour later, an assistant called her back to the set and handed her a new, historically accurate basket, so Ford could shoot the scene with her in it.

Included among the "1001 Movies You Must See Before You Die", edited by Steven Schneider.

Although Alfred Newman's justifiably highly-praised score utilized many Welsh melodies as well as the services of virtually every Welsh singer then living in Southern California, his principle love theme was based on the traditional Irish folk song "The Sixpence," not on anything Welsh.

Alexander Knox was Fox's first choice for the part of Dr. Gruffydd, later played by Walter Pidgeon.

The novel was adapted as a Broadway musical, called A Time for Singing, which opened at the Broadway Theatre, New York, on May 21, 1966. Music was by John Morris; book and lyrics by Gerald Freedman and Mr. Morris. Production directed by Mr. Freedman, starring Ivor Emmanuel, Tessie O'Shea, Shani Wallis, and Laurence Naismith.

Dennis Hoey is in studio records as being in the film but does not appear and is not credited in the final product.

Initially, John Ford had Rhys Williams record the narration as the adult Huw Morgan. When he became concerned that audiences would recognize the voice as belonging to Williams's on-screen character, the boxer Dai Bando, he had actor-director Irving Pichel re-record the lines, which had to be read in the same rhythms as Williams's to match sequences already cut to the narration. At one point, British prints of the film featured Williams's voiceovers.

"The Screen Guild Theater" broadcast a 30-minute radio adaptation of the movie on March 22, 1942, with Sara Allgood, Donald Crisp, Roddy McDowall, Maureen O'Hara and Walter Pidgeon reprising their film roles performing with Rhys Williams. This was followed by a 60-minute performance on September 21, 1942, with all the same except for Williams. Years later, Crisp repeated his role in radio productions of the same on March 31, 1947, and September 28, 1954.

When Huw is leaning against Mr. G after he's learned to walk again, there is a real fly that lands on Mr.G's hand. Not planned.

Fox technicians turned an 80-acre plot in Brent's Crags, near Malibu, into the Welsh mining village of the story. By that time, the budget had been scaled down to $1.25 million, which didn't allow for any shooting on actual locations. With the start of World War II in Europe, shooting overseas was almost impossible for the Hollywood studios anyway. Darryl F. Zanuck also abandoned his plans to shoot in Technicolor, partly because the Malibu countryside would not match the colours of Wales.

In 1966 a musical version of the movie entitled "A Time for Singing," was produced by Alexander H. Cohen with music by Mel Brooks collaborator John Morris, lyrics by Gerald Freedman and John Morris, and a book by Freedman and Morris. Although Cohen was enthusiastic about the project, calling it the best musical he had ever produced, it closed after 41 performances. Bing Crosby did record two of the songs for Reprise records.

Included among the American Film Institute's 1998 list of the 400 movies nominated for the Top 100 Greatest American Movies.

In 1939, Frank Capra was the first person to win 3 Academy Awards for Best Director. John Ford tied this record for this film and won a 4th for "The Quiet Man" - a record that still stands in 2019.

Features the only Oscar nominated performances of Sara Allgood and Donald Crisp (for which he won Best Supporting Actor).

This film was selected into the National Film Registry in 1990 for being "culturally, historically, or aesthetically significant"

The only film directed by John Ford to win Best Picture at the Academy Awards.

The birds that land on the windowsill when Roddy McDowall is ill in bed are probably Audubon's Orioles, a North American species.

The title of the novel appears in two sentences. It is first used in Chapter XXX, after Huw 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."

Cyfartha's final line, "'Tis a coward I am, but I will hold your coat," was added by John Ford over the objections of screenwriter Philip Dunne.

The author continued the story about Huw Morgan's life in three sequel:s. "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.