Zhemgang Dzong is situated atop the peak of a triangular shaped ridge that rises sharply from the Mangdechu, facing the village of Trong and the town of Zhemgang.

The founding of the Dzong is credited to Lama Zhang Dorje Drakpa who lived in the 12th century A.D. According to oral information, Lama Zhang, from Zhamling in Tibet, was a renowned scholar-sage of the Drukpa Kagyu school of Buddhism. In his mission to spread Buddhism in Bhutan, in 1163 A.D. he traveled as far as present Zhemgang, where he resided at the site of the present Zhemgang Dzong. Lama Zhang is considered by many to be the greatest Buddhist saint to have settled in Zhemgang. His importance in Kheng is justified by the fact that he was the founder of the Dzong, the most important religious building in the Kheng area.

Khenrig Namsum is the ancient name of Zhemgang Dzongkhag. It literally means the three divisions of Kheng: upper (Chikhor), Middle (Nangkor), and lower (Tamachok) Kheng. Later, around 1655 EC, on the site where Lama Zhang had previously built a hermitage, a one-storied Dzong was built to mark the unification of the Khenrig Namsum and defend the land against invaders led by Choetse Penlop.