Historically, the area was Mueang Wiang Kaen, located on the Ngao river bank. It was founded in the same time of Sukhothai kingdom and Chiang Rai.
The government separated the three tambon Muang Yai, Po and Lai Ngao from Chiang Khong district to create a minor district (King Amphoe) on April 1, 1987. It was upgraded to a full district on September 7, 1995.
The district is subdivided into 4 subdistricts (tambon), which are further subdivided into 41 villages (muban). There are no municipal (thesaban) areas, and 4 Tambon administrative organizations (TAO).