Leonid (in wartime documents he was referred to as Lev) Roizen was born in 1921 in Proskurov (now Khmelnitskii), Ukraine. Prior to the Soviet-German war, he lived in Odessa where, in 1940, he was drafted. Roizen was on active service from the first day of the war to its last, fighting with the 15th Guards Division. His military specialty was that of radio operator, and, perhaps, because of this relatively "peaceful" job, he survived all the four years of warfare, including the Battle of Stalingrad. In 1944 and 1945, his division fought in Poland, Germany, and Czechoslovakia. Roizen, who at this time served in an anti-tank battalion, was wounded three times. In Poland he witnessed the heroic deaths of his Jewish comrades in arms, Private Iakov Grinberg and deputy commander of the 47th regiment for political matters Iefim Glikman, to both of whom he devoted one of his postwar essays. Roizen ended the war in Czechoslovakia. He was awarded two Soviet military orders and a Czechoslovakian one, and also some medals.
Roizen's father and three brothers also fought in the Soviet-German war; two of the brothers returned from the war as invalids.
After the war, Roizen graduated from a military political school in Leningrad and continued his military service as a political officer. He retired in the late 1960s with the rank of captain and settled in Leningrad.