At the outbreak of the World War I, Maček was a reserve officer. As such, he was mobilised into 25th People's Regiment of the Austro-Hungarian Army as a commander of the 3rd company on 27 July 1914. Maček participated in the Serbian Campaign and was wounded in the Battle of Kolubara while crossing the river Kolubara in November. After returning from hospital in Novi Sad to Zagreb before the Christmas, he was decorated for bravery and promoted to the rank of first lieutenant. Due to his astigmatism, he was declared unfit to serve on the battlefield, and was appointed a commander of an engineer company, composed of Poles and Ukrainians. His company prepared defenses of Budapest, and later Austrian-Hungarian port in Pula, where he served until autumn 1916. From 15 October 1916 until 15 March 1917 he served in occupying forces in Albania.