Lancashire Constabulary is concerned for the welfare of the 15 year old who is missing from home in Lancaster and has links to Oldham.
Millie was last seen in the Langdale Road area at around 14:30hrs on 22 July.
Millie was wearing a grey and black UnderArmour top, jeans and white Nike shoes.
She is described as being white and 5ft 6 in tall. She has plum coloured hair.
Aside from Lancaster and Oldham, Millie also has links to Blackpool and Bishop Aukland, Scotland.
PC Dave Pinnington said: “Millie has been missing for three days now and our concern is growing. If you have seen her or know where she is, please get in touch with us as soon as possible.
“Millie – if you are reading this, you are not in any trouble. We just want to know you are okay.”
If you can help please call 01524 596920, 01524596927 or 101 quoting log number 1636 of 22 July.