Konstantin Zhostov was born in the village of Gaytaninovo, at the time part of the Ottoman Empire.

He was the son of Andon Zhostov, a teacher and a significant figure in the struggle for the autonomy of the Bulgarian Church.

In 1897 he graduated with excellent marks and returned to Bulgaria and was assigned to the administration of the Army's General Staff.

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.

After a few months of fighting the Ottoman Empire was finally defeated and the London Peace Treaty was signed on 30 May 1913.