Wat Yai Chai Mongkhon (Wat Chai Yai Mongkol) dates from the reign of King U Thong (Uthong) and was built around 1357. U Thong or Ramathibodi I (1314-1369) was the first king of the kingdom Ayutthaya; reigning from 1351 to 1369. He was known as Prince U Thong before he ascended to the throne on March 4; 1351. A native of Chiang Saen (now in Chiang Rai Province) he claimed descent from Khun Borom and propagated Theravada Buddhism as the state religion.

Ayutthaya (Ayudhya)) was a Siamese kingdom that existed from 1351 to 1767. Ayutthaya was friendly towards foreign traders; including the Chinese; Vietnamese (Annamese); Indians; Japanese and Persians; and later the Portuguese; Spanish; Dutch and French; permitting them to set up villages outside the city walls. In the sixteenth century; it was described by foreign traders as one of the biggest and wealthiest cities in the East. The court of King Narai (1656-1688) had strong links with that of King Louis XIV of France; whose ambassadors compared the city in size and wealth to Paris.