Born in Tigveni, Argeș County, he was the son of a teacher. He attended Saint Sava National College in Bucharest, the school for soldiers’ sons in Iași and the Bucharest military school, from which he graduated in 1907 as head of his class, with the rank of second lieutenant. After being promoted to lieutenant (1910) and then captain (1915), Marinescu served during the Romanian Campaign of World War I in a "Vânători de munte" regiment. Promoted to major in April 1917, he distinguished himself in the Battle of Mărăști. For his valor displayed at engagements at Topești and Bârsești in the Putna River valley, on 11 August 1917 he was awarded the Order of Michael the Brave, 3rd class. In 1918 he became commanding officer of the 9th Mountain Troops Regiment. After the war, he joined the staff of the rural gendarmerie, becoming lieutenant colonel in 1921 and colonel in 1926.