Mohamad Taher Siala, a foreign ministry employee for three decades, "officially took up his post" at the ministry building, Libyan news agency LANA reported.
He is the latest member of the embattled GNA to take control of a ministry building in the divided oil-rich country since May 25, following those of health, interior and national education.
The UN-backed GNA has taken control of several administrative offices over the past two months.
A rival government in the east is refusing to cede power until a repeatedly delayed vote of confidence is passed in Libya's elected parliament.