The son of Ukrainian peasants, Semyon Pavlovich Sakhnov was born on 15 February 1900 in Samoylovka in the Saratov Governorate of the Russian Empire (now Saratov Oblast, Russia). He graduated from the village primary school. Drafted into the Red Army on 20 July 1919 during the Russian Civil War, he was sent to serve with a cavalry battalion of the 23rd Rifle Division on the Southern Front. With the latter, Sakhnov fought as a Red Army man in the suppression of revolts in the rear of the 9th Army, and in battles against the Armed Forces of South Russia in the advance on Novocherkassk, on the Manych River, and at Yekaterinodar. He was transferred to the 99th Railroad Detachment at Balashov in February 1920 and later that year became a cadet of the 34th Machine Gun Commanders' Courses at Saratov, which became the Saratov Infantry School in May 1921. With a cadet detachment from the school, Sakhnov fought in the suppression of peasant revolts in Saratov Governorate.