He was head of secret laboratories in the Bach Institute of Biochemistry in Moscow (1928–1935). As the head of Laboratory No. 1 of the NKVD (1938–1946), he initiated the secret "scientific" poison program conducted by the Soviet secret police services. He used political prisoners for experiments with poisonsTemplate:Dubious. His classified PhD thesis defended in 1940 was entitled "Biological activity of the products of interaction of mustard gas with [human] skin tissues".
Mairanovsky participated personally in political assassinations as a member of the Pavel Sudoplatov team in the 1940s. These assassinations include that of American spy for the Soviets, Isaiah Oggins.
He was arrested in 1951, in connection with the arrest of Viktor Abakumov, and spent 10 years in prison. After his release, he headed a biochemical laboratory.