In the ranks of this division, Klevensky participated in the battle for Moscow, fought on the territories of the Moscow, Smolensk and Kalinin Regions; he also served as telephone operator and clerk of a battalion of the 129th Regiment of the 93rd Division of the 1st Formation. On 5 March 1942, already in the rank of sergeant, he was seriously wounded in the right arm near the town of Belyi (Kalinin Region). Because of the onset of gas gangrene, the arm was amputated over the elbow in a field hospital. An ambulance train took him to Moscow, where his family was residing, and where he was placed in evacuation hospital № 5005, supervised by the V.I. Lenin State Library of the USSR from the autumn of 1941 until the end of the war. Klevensky stayed in the hospital until the end of May 1942. In June 1942, he returned to the Lenin State Library and was appointed Academic Secretary.

For those who worked in the Library during the Great Patriotic War, Klevensky was a hero. He wrote letters from the frontline, wise and good letters that would support his colleagues in the Library in their difficult work of helping the front and rear.

For his valor on the battlefields, M.M. Klevensky was awarded the following orders: the one of the Patriotic War of the 1st Class, the Badge of Honour, sixteen medals, including: For Battle Merit, For the Victory over Germany in the Great Patriotic War of 1941-1945, For Valiant Labour in the Great Patriotic War of 1941-1945.

In 1930, Klevensky came to the Library to work as a library technician. Employees of the long-gone 30s, both those who had been working in the Library since the Moscow Public and Rumyantsev Museums, and the shock workers of the first five-year plans, considered him an endlessly dedicated person, in love with the Library. For he was doing and did so much that it would have been enough for several “library lives”.

He had become a chief librarian before the war. Over the years, he performed different jobs. Klevensky served as head of the Storage Department, Academic Secretary, and Deputy Director of the Library.

Severely wounded, Klevensky returned to his favorite Library. And he began to work again as best he could – with all the passion of a man in love with his work, a communist. In 1953, the first part of the “History of the State Order of Lenin Library of the USSR named after V.I. Lenin” was published, covering the period from its opening in 1862 to 1917; and to mark the 100th anniversary of the Library’s foundation, he wrote the fundamental work “History of the V.I. Lenin State Library of the USSR for 100 Years” (Moscow, 1962). He trained the staff and showed the Library to most distinguished guests. Those who worked with him would never forget his interesting informative lectures, as well as his amazing voice.

Mark Mitrofanovich Klevensky kept working in the V.I. Lenin State Library of the USSR until the middle of 1977.