On 10 May 1916 he was promoted to brigadier general, and on 15 August he took command of the 18th Infantry Brigade. After Romania entered World War I on the side of the Allies on 27 August, Mărdărescu was appointed Chief of Staff of the 3rd Army and then the 2nd Army. Under the command of General Alexandru Averescu, he helped coordinate the Romanian Campaign of 1916, first in the Dobruja, and then during the defense of the Predeal Pass. Mărdărescu was later put in charge of 3rd Army Group (a command he held until 1917), and fought at the battles of Mărăști and Oituz in July–August 1917. In February 1918 he advanced to divisional general.