The exterior shots were filmed at the former St. Mary's Home, an orphanage in Ambler, Pennsylvania. It is also known as Lindenwold Castle, or the Mattison Estate The main building looks like a stone castle and is still standing. It was featured prominently in this movie, but all of the interior shots were done in a studio in California.
Mary Wickes, who did not know how to swim, was ordered to take swimming lessons at the YMCA by Producer William Frye for the pool scene with Hayley Mills and June Harding. But for some reason, Wickes didn't take the lessons and wouldn't jump in the pool on the day of shooting. Director Ida Lupino, furious with Wickes, called Frye and he suggested they hire a stunt double to play Wickes from the back and have the double jump in the deep end of the pool. Wickes is only seen in close-up at the shallow end of the pool and had to be escorted in and out of the water by assistants. This incident cost the production several thousands of dollars and put them behind schedule.
Producer William Frye personally offered his friend Greta Garbo one million dollars to play Mother Superior. When he was unsuccessful, he turned to another MGM alumna. Rosalind Russell had been off the screen since Gypsy, vénus de Broadway (1962). A devout Catholic, Russell was an ideal choice for the nun, not only because of her familiarity with Catholic ritual, but also because of her air of authority and sense of humor.
According to Rosalind Russell, she and Hayley Mills did not get along during filming. She claimed that every time she turned around or walked away from Mills, she would stick her tongue out at her.
This was the final theatrical movie directed by Ida Lupino. She finished her directing career working in television.