In 1907 Mecenseffy was part of the group that worked on the "Plan U" which prepared for a possible occupation of the Habsburg Kingdom of Hungary in case of a rebellion.

[1][2] When World War I broke out in August 1914, Mecenseffy served as the chief of staff for the 2nd Army when it was initially deployed to be used against Serbia.

His unit saw action in the Gorlice–Tarnów Offensive against the Russian Empire, taking part in defending the flank of 3rd Army as it advanced towards Przemyśl fortress, before being transferred to the Italian Front, where it was also involved in heavy fighting.

His new unit held the line near Vil Sugana valley (Trentino) in the mountains, repulsing Italian attempts at breaking through.

On the personal insistence of the former Chief of the General Staff, Franz Conrad von Hötzendorf, his body was returned to his family in Vienna.