[3] Private Mestrovich was deployed along the Mexican border in support of the 1916 Punitive Expedition with the Pennsylvania National Guard, where his skill and experience as a soldier saw him promoted to corporal.

[3] Mestrovich was interviewed by newspaper reporters and attributed his patriotism and service as a debt repayment for the work of American doctors treating the typhoid epidemic in his native Serbia in 1914, the same year World War I began in Europe.

On August 10, 1918, while his unit was engaged in the town of Fismette, France, Sergeant Mestrovich saw his company commander, Captain James Williams, fall wounded as they moved through the ruins of the city.

Without regard for his own safety, Mestrovich charged forward through a hail of machine-gun fire and falling artillery shells to rescue Williams, returning to a concealed position to provide life-saving first aid.

"[7] He did recover and return to the 111th Infantry, but as fighting raged in the Meuse Argonne, Sergeant Mestrovich fell in action on November 4, 1918, with nearly 50 other men from the 111th, when their battalion encountered a concealed machine-gun position during a reconnaissance patrol just a week before the Armistice with Germany ended hostilities.