Aleksić was born in the Gornja Crnuća village within the municipality of Gornji Milanovac, Kingdom of Serbs, Croats and Slovenes. During World War II, when Axis powers occupied the Kingdom of Yugoslavia, 17-year-old Mija Aleksić attended high school in Kragujevac. On 21 October 1941, as a retaliation to sporadic Partisan guerrilla attacks in the area, German occupational authorities committed one of the worst massacres in regional history (Kragujevac massacre) – shooting several thousand men and boys of Kragujevac, including entire high school classes. Mija Aleksić managed to escape and was one of the few surviving men of his generation in his native town.