Microwave popcorn according to package directions. Line a rimmed baking pan with nonstick foil. Spread popcorn onto foil-lined pan.
Melt butter in a small saucepan over medium heat. Add sugar and corn syrup. Increase heat to high and bring to a boil, stirring constantly. Boil until sugar dissolves, about 2 minutes. Cool for 4 minutes.
Pour sugar mixture over popcorn on baking pan. Coat a spatula with nonstick cooking spray. Push hot mixture into center of baking pan, folding over to combine. Coat your hands with nonstick cooking spray and sprinkle popcorn mixture with M&M's. Using your hands, quickly combine ingredients and shape into firmly packed 2-inch balls. Cool completely. Wrap each ball in plastic wrap or keep covered until ready to serve.