In 1916, at the beginning of the Romanian campaign of World War I, Mociulschi was in command of the 10th Company of the 29th Infantry Regiment (Dorohoi), holding the rank of lieutenant. Mociulschi participated in the Battle of Transylvania with the North Army and fought at the First Battle of Oituz in the fall of 1916. In the summer of 1917 he fought at the Battle of Mărășești with the Romanian 2nd Army. For his valor during the Oituz and Soveja battles he was decorated by King Ferdinand I of Romania and General Henri Mathias Berthelot, the commander of the French Military Mission in Romania, and promoted to the rank of captain.