He was born in Odessa, Russian Empire, as Semyon Kesselman. In 1914 Zapadny graduated from the Jewish treasury school in Odessa. In July 1917 Zapadny joined the Odessa Red Guards (Bolshevized paramilitary formations) and later around the October Revolution joined the Bolshevik Party. Zapadny participated in the Odessa Bolshevik uprising in December 1917. With the ongoing Soviet–Ukrainian War, he joined the Workers'-Peasants' Red Army. After signing of the Treaty of Brest-Litovsk and occupation of Ukraine by the Central Powers, in April 1918 to August 1919 he was one of leaders of the Odessa underground Bolsheviks committee on intelligence and counterintelligence (Odessa Cheka was headed by Boris Severny [Yuzefovich]). Sometime in 1919 Zapadny officially joined Cheka, becoming member of the Odessa Governorate Cheka collegium. He remained among the high-ranking Cheka and GPU officers in Ukraine until 1930.