Chocolate milk was invented by Sir Hans Sloane, an Irish botanist, in the 18th century. While working in Jamaica, the local people served him cocoa to drink, but he was not fond of the taste. He mixed the cocoa with milk and found it much more palatable.
Sloane took the milk-and-cocoa tradition home with him to England and even sold it as medicine. He was not the only one to bring a cocoa drink to his home country: Cortez was served cocoa by the Aztecs and brought the recipe back to Europe in the 1500s. And the native Jamaican people who first served Sir Sloane had been brewing their own version of hot chocolate, with boiled milk and cinnamon, since at least 1494.