In 1915, Dimăncescu joined Prince Carol's Hunters Regiment. From August 1917 until the Armistice of 1918, he served as an officer, rising to the rank of captain. He aided British intelligence officers in the sabotage of Romania's oil wells and depots in December of 1916 and participated in the Third Battle of Oituz of August 1917. He led his company against Rommel's German forces on Cosna Mountain. He was rewarded with the British Military Cross, the Virtutea Militară, the Star of Romania, the Order of St. Stanislas, Furajera Medal of the Order of Mihai Viteazul, and Commander of the Royal Victorian Order.