An Irish physician named Sir Hans Sloane is the inventor of chocolate milk. He came up with it while he was traveling through Jamaica.
He went to Jamaica in 1687 to become the physician for the Duke of Albemarle. The only problem was that the Duke died soon after landing, and his visit lasted only 15 months. During that time, however, he noted about 800 new species of plant, and the most important thing of all, he encountered cocoa.
Locals in Jamaica drank cocoa with water, something that Sloane found nauseating. He saw promise in the drink, however, and decided to find ways to make it more acceptable. He struck gold when he managed to mix it with milk.
He brought back his chocolate milk recipe with him back to England, where it was manufactured and sold by apothecaries as medicine.
The rest is history, delicious, chocolatey history.