James was born as Joko Mestrovic in the Bay of Kotor, Austrian Littoral, which is now Montenegro, on the 22nd of May, 1894, and he emigrated to the United States in 1913. He moved from Fresno, California to Pennsylvania where he enlisted in the US Army in Pittsburgh with the 28th Infantry Division. James was a Sergeant with Company C, 111th Infantry when he was deployed to France and it was his actions on the 10th of August, 1918, that earned him the Medal of Honor. The citation reads:

Seeing his company commander lying wounded 30 yards in front of the line after his company had withdrawn to a sheltered position behind a stone wall, Sgt. Mestrovitch voluntarily left cover and crawled through heavy machinegun and shell fire to where the officer lay. He took the officer upon his back and crawled to a place of safety, where he administered first-aid treatment, his exceptional heroism saving the officer's life.

James continued to fight and serve after these actions, but contracted the Spanish Flu, which took his life one week before the end of the War on the 4th of November, 1918. He posthumously received the Medal of Honor on the 31st of December, 1919, and James I Mestrovitch is buried in the Sveti Jovan Churchyard in Kotor, Montenegro.