Following the death of his father in 666, Namgeon became embroiled in a power struggle with his elder brother Yeon Namsaeng.
Namgeon and Namsan's coup forced Namsaeng to surrender to the Tang, and ultimately led to the destruction of Goguryeo.
Namgeon appointed himself Dae Mangniji (대막리지, 大莫離支) and seized control of the government.
During Tang's subsequent invasion of 668, Namgeon led armed resistance to the Chinese forces near the Amnok River and continued to resist the Tang forces all the way to Pyongyang.
With Goguryeo's defeat, Namgeon found himself a prisoner of war and was transported to China and later banished to Qianzhou (黔州), in what is today China's Sichuan province, where he apparently died.