He started his career serving under his more illustrious brother Zakhar Chernyshyov at the Russian missions in Copenhagen (1741) and Berlin (1742–45). In 1749 he was commanded to resign from diplomatic service and marry Countess Elizabeth Yefimovskaya, a cousin of Empress Elizabeth. All three Chernyshov brothers backed Catherine in the coup that placed her on the Russian throne in 1762, after the assassination of her husband, Peter III. They were handsomely rewarded for their loyalty.