My fondness for putting milk in tea comes from a childhood filled with homemade chai, and Sleepytime tea from Celestial Seasonings both with copious amounts of milk. But how did the idea occur to the first people to try it? and who were those people?
Despite the stereotype that tea (and milk in tea especially) are quintessential British, both the Dutch and the French were first in both regards.
The Dutch had been drinking tea since 1610, and were probably putting milk into it all along. However, the first written mention of it was not till 1680 in a letter written by Marie de Rabutin-Chantal, marquise de Sévigné to her daughter. Marie was French aristocrat known for her prolific letter writing.
Its no surprise that milk found its way into tea so quickly. The Europeans are the highest consumers of milk in the world, with the average person drinking about 33 gallons a year! (Compared to America's 20 and China's 2 gallons a year)
Even though the British did not start the tradition of tea drinking, or adding milk, they certainly popularized it around the world though their colonial influence. In India almost everyone drinks chai, which is made without water there. As a result the average Indian drinks about 25 gallons of milk a year.
Hong Kong also has a long tradition of drinking "milk tea", which is made from Ceylon black tea, evaporated milk, and sugar (or condensed milk). The idea is to get a froth that forms around the inside lip of the cup from the milk fat; more fat equals more froth. Despite most Chinese being lactose intolerant 900 million cups are served a year!