The former Made in Chelsea star made the admission in the show's official handbook, and has revealed that she was a bit of a rebel in her teenage years.
"I was a little devil at school," said Millie, writing in Made in Chelsea: Life and Style Essentials. "I went to five different schools because I was really rebellious and I was just determined to be as naughty as possible."
The 23-year-old fashionista, who has a younger sister Alice, wasn't always as close to her sibling as she is now. It seems Millie's naughty phase started at the tender age of six.
"I hacked off my little sister's hair," said the on-screen star. "I went for her with a pair of kitchen scissors the day before we were due to be bridesmaids. I was only six years old and I don't even know why I did it."
Millie, who recently married rapper Professor Green — real name Stephen Manderson — in September, looked stunning on her wedding day and has always had a reputation for dressing the best on the show.
"I've always been a clothes horse," added Millie. "My sister Alice would put on a lot of Disney princess costumes, but I'd go straight for my mum's old clothes — some really great 80s Versace flame prints, miniskirts and heels that were way too big."
Shortly after her honeymoon with Professor Green, the stylish make up artist denied rumours that she was pregnant.
"Pregnancy rumours are just that," said Millie. "We are far too busy enjoying each other right now".