Semyonov was born Samuil Markovich Taubman in Odesa and graduated from the Moscow Textile Institute in 1936 with a specialty in power engineering.
[citation needed] Of Jewish ancestry, for which he would suffer career-wise in the Soviet Union following the Stalinist purges, Semyonov joined the NKVD in 1937 and was sent quickly to the United States as an intelligence officer.
[citation needed] In 1942 Semyonov persuaded Vasily Zarubin to transfer Julius Rosenberg and his contacts from the CPUSA-Jacob Golos channel to the direct control of the Rezidentura, with himself as the assigned case officer.
[citation needed] Semyonov persuaded into collaboration a large group of young scientists and specialists, through whom was obtained a significant quantity of valuable materials on "ENORMOZ" (Manhattan Project), radio electronics, jet aviation, chemistry, medicine.
[citation needed] In 1950, in association with the Doctors' plot, the foreign intelligence agency began dismissing persons of Jewish ethnicity.