Shimon (Semion) Gendel was born in Novo-Borisov, Belorussia in 1903 into a poor Jewish family. As a small child Shimon attended heder (traditional Jewish elementary school). His older six brothers and sisters immigrated to the United States before the Bolshevik Revolution of 1917. After graduating from a Soviet 7-year school, Shimon left for Leningrad, where he worked as a stevedore in the city's commercial port. In 1926 Shimon Gendel was drafted into the Red Army and, after completing the Signal Corps School, served as a communications officer. After his discharge in 1927, he worked for several years at the Putilov Factory (since 1934 called the Kirov Factory) in Leningrad. Later Gendel entered the Leningrad Engineering Institute of Waterway Transport but in 1932, after two years of study, he decided to pursue a military career and rejoined the Red Army. After completing an officer training course, by then the head of a signal regiment, Gendel participated in the Winter War (December 1939 - March 1940) between the Soviet Union and Finland.
After the outbreak of the war between Soviet Union and Nazi Germany on June 22, 1941, Lieutenant-Colonel Semion Gendel was assigned to be commander of a communications unit on the Leningrad Front. Later he fought on the 2nd and 3rd Baltic Fronts. As a communications officer, Gendel provided Red Army staff at headquarters and on the battle field with ongoing communication via telephone and radio. He also maintained communication lines for army operations. Having been a member of the Communist Party since 1927, he also served as a political commissar (politruk) of his regiment.
The article "The biography of Lieutenant-Colonel Shimon Gendel" by Shmuel Persov, prepared in 1944 by the Jewish Anti-Fascist Committee to be sent to the Jewish press in the West, described Gendel's mission as a communications officer. The article stressed Gendel's Jewish background by referring to him by his Jewish name Shimon. It also noted that the future Red Army officer was raised in a traditional Jewish environment.
Lieutenant-Colonel Semion Gendel ended the war in Courland, Latvia. During the war he was awarded the Order of the Red Star, the Order of the Red Banner, and after the war had ended, in June 1945 the Order of the Patriotic War, 2st class.