He advanced in rank to captain in 1896, and on 10 May 1906 he was promoted to major and named commandant of the Infantry Shooting School.

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.

For his actions during the war he was awarded by King Ferdinand the Order of Michael the Brave, 2nd Class.

In August 2019, on the occasion of the 100th anniversary of his victory in the Hungarian–Romanian War, a statue of Mărdărescu was unveiled on the campus of Babeș-Bolyai University in Cluj-Napoca.