Archeological evidence suggests that northwest Thailand was where prehistoric humans first learned to cultivate food plants. In historical times, before the Thais settled in the region, it was inhabited by a Mongoloid people called the Mons. In the first century A.D. the Mons came under the rule of the great commercial kingdom of Funan, centered in Cambodia. The Mons overturned Funan's rule in the sixth century and founded a kingdom in the Chao Phraya valley. In the 10th century, the Khmers (Cambodians) established themselves in Thailand and made the Mon kingdom a vassal state of their powerful empire.

Thais from southern China began settling in northern Thailand in the late 11th century. By the mid-1200's, many Thais had migrated farther south to the upper Chao Phraya valley and had formed small principalities within the Khmer Empire. In 1253 the Mongols conquered the Thai settlements in China and hundreds of thousands of Thais fled south into the valleys of the Mekong and Chao Phraya rivers.