Semeyko was born on 25 March 1923 to a working-class Ukrainian family in Sloviansk. He completed nine grades of school and graduated from an aeroclub before entering the military in August 1940. There he trained in aviation, and in February 1943 he graduated from the Voroshilovgrad Military Aviation School of pilots, which was based in Kazakhstan away from the front of the war. He became a member of the Communist Party in 1943.
