Slepnyov was born on June 15 (27), 1896, in Yamskovitsy village, Yamburgsky Uyezd, Saint Petersburg Governorate (now Kingiseppsky District, Leningrad Oblast) in a peasant's family.

In 1925, Mavrikiy Slepnyov became a pilot of the Civil Air Fleet and took part in the exploration of the airways of Central Asia, the Russian Far East, and the Arctic.

[1] Slepnyov was awarded the title of the Fifth Hero of the Soviet Union on April 20, 1934, for the rescue of the SS Chelyuskin crew from an improvised airfield on the frozen surface of the Chukchi Sea near Kolyuchin Island.

In 1937, he was appointed Chief Inspection Head at the Civil Air Fleet and dirigible squadron commander, at the same time (since 1935).

During the German-Soviet War, Slepnyov was a deputy commander of the Soviet Air Force Black Sea Fleet aviation brigade (1941–1942) and then was employed by the Main Department of the Naval Air Force and General Headquarters of the Soviet Navy.