- Creator Breakdown: Beatrice Lillie was showing the early signs of Alzheimer's disease and had trouble remembering her lines. Julie Andrews would stand off-camera and repeat her lines for her so Beatrice could finish the scenes. This would be her last film.
- Cut Song: Mary Tyler Moore claims she originally had a solo song that got cut from the film.
- The Danza: James Fox plays James van Hossmere. Ditto for his singing voice Jimmy Bryant.
- Executive Meddling: Miss Dorothy was originally the main character. When Julie Andrews was cast, the studio had her role beefed up so that Millie was now the lead.
- Fake Nationality: Pat Morita and Jack Soo were both Japanese playing Chinese henchmen. Philip Ahn was Korean.
- Mid-Development Genre Shift: Originally conceived as a light comedy to help make Mary Tyler Moore the "next Doris Day", when Julie Andrews came on board it was changed to a musical.
- Non-Singing Voice: Jimmy Bryant dubbed James Fox for his singing.
- Real Life Writes the Hairstyle: The opening montage features Millie deciding to bob her hair as she starts her new life. Julie Andrews herself had long hair on the stage but bobbed it as she filmed Mary Poppins.
- Suppressed Mammaries: A rare adult example: Julie Andrews did this in order to achieve The Roaring '20s-appropriate pettanko "flapper" look and lampshaded it within the film.
- What Could Have Been: The studio wanted Robert Wagner for Jimmy, but he was played by James Fox.
Trivia / Thoroughly Modern Millie