Fa Ngum was a grandson of Souvanna Khamphong, ruler of Xieng Dong Xieng Thong and descendant of Khun Lo. He was exiled as a child to the court of Angkor along with his father. When he decided to return he brought an army of 10,000 with him and proceeded to conquer much of peninsular Southeast Asia.
In the decade 1340-1350, he conquered the Khorat Plateau, including Udon, Surin, Champassack, and Nongkhay. In 1351 he conquered Mouang Phouane, Nge-Anh, and Vinh as well as the Red and Black River valleys in Vietnam. In 1352-1353, he conquered Muang Sing, Muang Huom, Xieng Hung, Pak Ou and Pak Beng. In 1353, he fought the ultimate battle against his uncle near Xieng Dong Xieng Thong and won, becoming the undisputed master of the land, which he named Lan Xang.
He continued to expand his empire in 1354, pushing into Pak Beng, Pak Tha, Xieng Khong, Xieng Sene (Chiang Saen), and Xieng Hai (Chiang Rai). By 1358, the boundaries of his kingdom were recognized as Simao in the north, Sambor in the south, Khorat in the west and the Red River in the east. Finally in 1373, the royals and notables exiled him to Nan where he died and was enshrined in the Xieng Ngam temple. His son Samsenethai then ascended to the throne of Lan Xang.