Holy shit. TIL.

Wallace ended up buying a William Shatner Star Trek mask from a costume shop for under two dollars. To remove the human element of the mask, Wallace widened the eye holes and got rid of the side-burns and distinctive eyebrows. The mask was then spray-painted, transforming it into the emotionless figure that fans now know so well.

https://screenrant.com/halloween-michael-myers-mask-origin-story/