This was Robert Donat's last film, he died during its making. In the scene where he is saying goodbye to Gladys as the elders prepare to take their leave of the city, he says as though he was prophesying his death, "I fear we shall never see each other again."
The real Gladys Aylward (1902 - 1970), born in London, was a former domestic turned missionary in China, best known for her work with children. She became a Chinese citizen in 1936. Four years later, despite being in ill health herself, she shepherded over 100 children over the mountains to safety at the height of the Sino-Japanese war. In 1958, the year this film was released, she founded a children's home in Taiwan, which she continued to run until her death. Known in China as "Ai-weh-deh", or "Virtous One", she continues to be regarded as a national heroine.
The outdoor scenes were almost entirely filmed in North Wales, near where the Prisoner was set. Some studio work was also done in England, meaning the entire film was made in Western Europe using Chinese extras. Most of the children in the film were Chinese children from Liverpool, which has one of the oldest Chinese communities in Europe.
According to director Robson, Donat kept working until the end although he was very ill. After finishing the dubbing, Robson urged him to go home, but Donat wanted to get a haircut from the studio barber. The next day he was taken to the hospital for the last time.
The song "The Children's Marching Song", more commonly known as "This Old Man", was better-known in England than the U.S. at the time this film was made. When the film became a hit in America, so did the song, helped in no small measure by Mitch Miller's popular recording.