At the beginning of the Great Patriotic War of 1812 he made a speech before the members of the Moscow nobility which left an epic impression. The text has not been preserved, though Pushkin later dubbed it "immortal". Mamonov offered the government to use his whole income for the war effort keeping back just 10,000 rubles a year for his personal use; further, he also offered to mobilise his serfs. The tsar thanked him for his offer and suggested he raise a cavalry regiment within the Moscow militia.