When the general mobilization prior the First Balkan War was declared in September 1912 Konstantin Zhostov was appointed Chief of Staff of the 3rd Army and as such was at the heart of the Bulgarian victories at Lozengrad and Lüleburgaz.
During the First Balkan War (1912–1913) he was in command of the 3rd Army which decisively defeated the Turks at Lozengrad and Lule Burgas in Thrace.