Throughout the reign of Catherine the Great, Rumyantsev served as supreme governor of Little Russia. In this post, which his father had held with so much honesty, Rumyantsev made it his priority to eliminate any autonomy of the hetmans and to fully incorporate the newly conquered territories into the Russian Empire. Some accuse him of having promoted serfdom in New Russia, but the choice of such a policy remained out of his control. With the outbreak of the Russo-Turkish war in 1768, Rumyantsev took command of the army sent to capture Azov. He thoroughly defeated the Turks in the Battles of Larga and Kagula, crossed the Danube and advanced to Romania. For these dazzling victories he became Field-Marshal and gained the victory titleZadunaisky. When his forces approached Shumla in 1774, the new Sultan Abdul Hamid I started to panic and sued for peace, which Rumyanstev signed upon a military tambourine at the village of Küçük Kaynarca.
By that point, Rumyantsev had undoubtedly become the most famous Russian commander. Other Catharinian generals, notably Potemkin, allegedly regarded his fame with such jealousy that they wouldn't permit him to take the command again. In times of peace, Rumyantsev expressed his innovative views on the martial art in the Instructions, Customs of Military Service, and the Thoughts. These works provided a theoretical base for the re-organisation of the Russian army undertaken by Potemkin. During the Second Russo-Turkish War, Rumyantsev suspected Potemkin of deliberately curtailing supplies of his army and presently resigned his command. In the Polish campaign of 1794 he once again won appointment as commander-in-chief, but his rival Suvorov actually led the armies into battle. On this occasion Rumyantsev didn't bother even to leave his Ukrainian manor at Tashan which he had rebuilt into a fortress. He died there on 19 December 1796, just over a month after Catherine's death, and was interred in the Kiev Pechersk Lavra. As the story goes, old Rumyantsev-Zadunaisky grew enormously fat and avaricious, so that he pretended not to recognize his own sons when they came from the capital to ask for money. Under his son Sergey's administration, Tashan fell into ruins, although he erected a mausoleum near Balashikha for his father's reburial. Neither Sergey nor his brother Nikolay Petrovich Rumyantsev married, and the comital branch of the Rumyantsev family became extinct upon their death.