King Mangrai, the 25th ruler of Ngoenyang (now known as Chiang Saen), became a significant figure in unifying different Tai city-states in the Lanna region. After inheriting the throne in 1259, he recognized the disunity and vulnerability of the Tai states. To strengthen his kingdom, Mangrai conquered several neighboring regions, including Muang Lai, Chiang Kham, and Chiang Khong. He also formed alliances with nearby kingdoms, like the Phayao Kingdom.

In 1262, Mangrai shifted his capital from Ngoenyang to the newly established city of Chiang Rai, which he named after himself.^[5] The word 'Chiang' means 'city' in Thai, so Chiang Rai would mean 'the City of (Mang) Rai'. He continued his expansion southwards and took control of the Mon kingdom of Hariphunchai (now Lamphun) in 1281. Over the years, Mangrai changed his capital multiple times due to various reasons, such as flooding. He eventually settled in Chiang Mai in 1292.

During his reign, Mangrai was instrumental in fostering peace among regional leaders. In 1287, he mediated a conflict between King Ngam Muang of Phayao and King Ram Khamhaeng of Sukhothai, leading to a powerful friendship pact among the three rulers.^[5] However, his ambitions did not stop there. Mangrai learned about the wealth of the Mon kingdom of Haripunchai from visiting merchants. Despite advice against it, he planned to conquer it. Instead of direct warfare, he cleverly sent a merchant named Ai Fa to infiltrate the kingdom. Ai Fa rose to a position of power and destabilized the kingdom from within. By 1291, Mangrai successfully annexed Haripunchai, causing its last king, Yi Ba, to escape to Lampang.^[5]