Millie is a bubbly French engine who is owned by Sir Robert Norramby, who runs the narrow gauge line at Ulfstead Castle, where she lives and works. Her duties include assisting the groundskeeper and taking visitors on tours of the estate. For this, she has her own open-topped carriages. While the Earl was travelling the world, Millie stayed in her shed at the castle.
Millie once swapped jobs with Luke. Although she struggled to adjust to start with, she ended up loving the Blue Mountain Quarry, although she was pleased to return to her own job at the castle and even more pleased to have made friends with Luke.
While she was helping with construction of the Dinosaur Park, she was teased by Samson and Harvey when they thought she was a silly little engine. She decided to pay them back by asking the groundskeeper to make a bonfire by the volcano to create the illusion that it was actually active.
Millie is a narrow gauge engine. She has worked on the Earl of Sodor’s Estate Railway for many years. She is very loyal to the Earl and her relationship is like that of a personal servant. They have fun together and are as close as many friends, but there is also a certain formality to their relationship. Millie is a youthful, fun-loving, friendly engine who is full of spirit, and can be fiery when crossed. But she is also fair-minded and never spiteful. That said, she will take it upon herself to teach an uppity engine a lesson, should she feel the need. She always wants to do the best by the Earl and will defer to his judgement in all things, but when it comes to other engines, she has a mind of her own.
Millie is based on Decauville 0-4-0 well tank No. 8069 "Tabamar", built in 1911. Unlike her basis, Millie has a rear coal bunker. Her and her basis have rear buffers but none in the front.
Millie is painted French blue with white lining, a golden running board, boiler bands and wheel spokes, Sir Robert Norramby's crest on her cab sides and her name painted in white on the sides of her tank.
- Season 17 - The Switch, Too Many Fire Engines (cameo), Santa's Little Engine (cameo) and The Afternoon Tea Express (cameo)
- Season 18 - Missing Gator, Marion and the Dinosaurs, Samson at Your Service (cameo) and Millie and the Volcano
- 2014 - The Switch (speaks in speech bubble only) and Leaves on the Line! (does not speak)
- 2015 - Marion and the Dinosaurs
- Miranda Raison (UK/US; King of the Railway onwards)
- Teresa Gallagher (DC Super Friends™ MINIS Mash Ups Origin Story!)
- Sonja Stein (Germany)
- Anna Gajewska (Poland; King of the Railway - seventeenth season)
- Beata Jankowska-Tzimas (Poland; eighteenth season onwards)
- Minako Saito (Japan)
- Sarah MacDonald Berge (Norway; King of the Railway onwards, excluding the seventeenth season)
- Marit Berg (Norway; seventeenth season only)
- Blanche Ravalec (France and French speaking Canada)
- Annie Rojas (Latin America)
- Tillie Bech (Denmark)
- Paqui Horcajo (Spain)
She plays Fauna in Sleeping Beauty (ThomasTenCents34526's Style)
- She is one of Princess Aurora's friends.
- Millie has the same whistle sound as Lady.
- She is the first female narrow gauge engine in the series.
- Millie is incorrectly portrayed as an 0-4-2 saddle tank engine in Thomas Land US.
- Wooden Railway
- Take-n-Play (normal and dusty)
- Wind-up Trains
- Minis (classic, robo, warriors, DC super friends (Harley Quinn and Catwoman) and SpongeBob SquarePants (Sandy Cheeks))
- Thomas and Friends Adventures (coming soon)