At age 19, he participated in his father's successful invasion of the city of Sukhothai, formerly a vassal of the Khmer, establishing the independent Sukhothai kingdom. Because of his conduct in the war, he allegedly was given the title "Phra Ram Khamhaeng" (Rama the Bold), though he is recorded in the Ayyutthaya Chronicles as King "Ramaraj". After his father's death, his brother Ban Muang ruled the kingdom and gave Prince Ramkhamhaeng control of the city of Si Sat Chanalai.
The Royal Institute of Thailand speculates that Prince Ram Khamhaeng's birth name was "Ram" (derived from the name of the Hindu epic Ramayana's hero Rama), for the name of him following his coronation was "Pho Khun Ramarat" (Thai: พ่อขุนรามราช). Furthermore, at the time it was a tradition to give the name of a grandfather to a grandson; according to the 11th Stone Inscription and Luang Prasoet Aksoranit's Ayutthaya Chronicles, Ram Khamhaeng had a grandson named "Phraya Ram", and two grandsons of Phraya Ram were named "Phraya Ban Mueang" and "Phraya Ram".