In 1763, Emperor Peter III sent him to Prussia as ambassador. The same year, Catherine transferred him to Poland as minister plenipotentiary; in Warsaw he was rumored to have had an affair with Izabela Fleming (and to have fathered Adam Jerzy Czartoryski).
In effect, due to the level of Russian control of the Polish government, he was an effective ruler of the country, with special instructions to form a pro-Russian faction from among the various Protestants, who were to receive equal rights with the Catholics. Repnin believed that the Protestants were not significant enough to benefit Russia; at the same time, the Protestant community itself petitioned Empress Catherine, requesting not to be involved.
In order to further Russian interests, he encouraged the creation of two Protestant confederations (of Sluck and Toruń) and later, a Catholic one (the Radom Confederation, led by Karol Stanisław "Panie Kochanku" Radziwiłł). According to the Encyclopædia Britannica Eleventh Edition, Repnin's correspondence reveals that he disliked the type of politics he was required to engage in. Nevertheless he obeyed his instructions, and used various means to force the 1767–68 Sejm (the "Repnin Sejm") to concede all points in dispute. Before the Sejm, he ordered the capture and exile to Kaluga of some vocal opponents of his policies — Józef Andrzej Załuski and Wacław Rzewuski. The immediate result was the formation of the Bar Confederation, which practically demolished the ambassador's strategy.
On the outbreak of the War of the Bavarian Succession he led 30,000 men to Breslau, and at the subsequent congress of Teschen, where he was Russian plenipotentiary, compelled Austria to make peace with Prussia.
During the second Turkish war (1787-92) Repnin was, after Alexander Suvorov, the most successful of the Russian commanders. He defeated the Ottomans at Salcia, captured the whole camp of the seraskier, Hassan Pasha, shut him up in Izmail, and was preparing to reduce the place when he was forbidden to do so by Grigori Alexandrovich Potemkin (1789). On the retirement of Potemkin in 1791, Repnin succeeded him as commander-in-chief, and immediately routed the grand vizier at Machin, a victory which compelled the Ottomans to accept the truce of Galaţi (July 31, 1791).
Repnin had an illegitimate son, Ivan Pnin, and it was widely rumored that Adam Jerzy Czartoryski was the fruit of a liaison between Repnin and Izabela Fleming. Repnin's legitimate children were three daughters. Upon his death, as the male Repnin line became extinct, Alexander I permitted Repnin's grandson Prince Nikolai Volkonsky to assume the Repnin name and his grandfather's coat-of-arms.