There is at least one historical mention of eating popcorn with milk, and it comes in Laura Ingalls Wilder’s Farmer Boy, page 32-33, a book set in the late 1850s. She mentions that young Almonzo (who would become her husband) liked popcorn and milk. “You can fill a glass to the brim with milk and fill another glass of the same size brim full of popcorn, and then you can put all the popcorn kernel by kernel into the milk and the milk will not run over. You cannot do this with bread. Popcorn and milk are the only two things that will go into the same place. Then, too, they are good to eat.” Of course, she also repeats the myth about Indians and Pilgrims and popcorn at Thanksgiving, so she is not wholly reliable. Even if we take her words at face value, she isn’t talking about breakfast cereal; she talking about a science experiment that tastes good.
The first packaged, ready-to-eat breakfast cereals were invented in the 1870s and made of oats and wheat. Cereal took a turn for the better in the early years of the twentieth century when the Kellogg brothers accidentally invented wheat flakes and corn flakes. None of these cereal pioneers used popcorn, presumably because it doesn’t work well.