Scientific historical literature highly estimates the activities of Russian party executives, Comintern members, military specialists, diplomats and Buryat national democrats to have played an essential role in the implementation of the Mongolian Revolution. However, those research works pay little attention to the life and deeds of O. I. Makstenek — Soviet diplomat and first authorized representative of the RSFSR People’s Foreign Affairs Commissariat to Mongolia — who had significantly contributed to the success of the Mongolian Revolution of 1921. Goals. The study seeks to identify and analyze archival documents about revolutionary efforts of O. I. Makstenek and his work as Soviet Russia’s authorized diplomatic representative to Mongolia contained in Russian and foreign depositories. The objectives to be solved thereto include as follows: 1) analysis of files of a criminal investigation against O. I. Makstenek from the State Archives of the Russian Federation, 2) analysis of the report titled ‘Outcomes of Activities of RSFSR Foreign Affairs Commissariat’s Plenipotentiary to Mongolia O. I. Makstenek from June 1, 1920 to March 1921’ and housed by Archives of Mongolia’s Ministry of Foreign Affairs, 3) introduction of documents about O. I. Makstenek from the State Historical Archives affiliated to the National Archives of Latvia (collections ‘Baltic District Security Department’, ‘Police Department of Riga Uyezd’, ‘Gendarme Department of Livland Uyezd’, 4) identification and analysis of documents stored at the Russian State Archive of Sociopolitical History, 5) exploration of archival documents about O. I. Makstenek’s activities in his diplomatic position at the Archive of Foreign Policy of the Russian Federation, 6) analyses of materials from the State Archive of Khabarovsk Krai. Conclusions. The archival documents show O. I. Makstenek as a notable historical figure whose life and deeds provide further details on revolutionary events during the 1900s to 1920s in Livland, Irkutsk Governorate, Transbaikalia and Mongolia, little-known aspects of confrontation between Makstenek and executives of the Far Eastern Republic, Troitskosavsk Communists, on strictness and inevitability of Party’s penalties throughout that period. It is his work as authorized representative of the RSFSR People’s Foreign Affairs Commissariat to Mongolia that can be viewed a pinnacle of his revolutionary activities when he made a most significant contribution to the victory of the Mongolian Revolution of 1921 — thereafter virtually to become a common Soviet functionary, mid-level executive in judicial authorities, industrial sphere, banking and customs services.