At the age of 19 he participated in his father's invasion of the city of Sukhothai, and was given the title "Phra Ramkhamhaeng", or Rama the Bold. After his father's death his elder brother Ban Muang ruled, giving Prince Ramkhamhaeng control of the city of Sri Satchanalai. On his accession, therefore, Prince Ramkhamhaeng had an established reputation for leadership.
Ramkhamhaeng formed an alliance with the Nguan Dynasty of China, from whom he imported the techniques for making ceramics now known as Sangkhalok ware. A story describes his seduction of the wife of King Ngam Muang, the ruler of neighbouring Phayao - an event which may have helped him to form his three-way alliance with Ngam Mueang and with King Mengrai of Chiang Mai, both of whose kingdoms were to the north of Sukothai. Ramkhamhaeng expanded his kingdom as far as Lampang, Phrae and Nan in the north, Phitsanulok and Vientiane in the east, Mon in the west, as far as the Gulf of Bengal in the northwest and Nakhon Si Thammarat in the south.
Ramkhamhaeng is traditionally credited with developing the Thai alphabet (Lai Sue Thai) from earlier Khmer, Mon and Burmese scripts, on the evidence of an inscription (the Ramkhamhaeng stele, now in the National Museum in Bangkok) dated to 1283 or 1292 which bears the earliest known Thai writing. He is also still respected as the king who introduced the style of benevolent monarchy that remains today.
His successor was Pho Khun Loethai.