Next year, tsar Alexander assigned Chernyshyov to be his permanent private envoy to Napoleon (parallel to regular Ambassador of Russia).
Chernyshyov, a member of Napoleon's retinue, accompanied French headquarters throughout the War of the Fifth Coalition, and received the Légion d'honneur for the Battle of Wagram.
Mutual understanding between Chernyshyov and Bernadotte became an asset in January 1812, during their negotiations about Sweden's stance in the anticipated French invasion of Russia.
However, Russian diplomacy failed in recruiting Bernadotte into military coalition, and the talks culminated in the Treaty of Åbo (30 August 1812), which upheld Sweden's friendly neutrality.
In the end of September 1812, Admiral Pavel Chichagov assigned Chernyshyov to lead a mobile detachment into the Duchy of Warsaw (south-eastern Poland), to destroy enemy food dumps and disrupt French supply lines.
In the end, Austrians intercepted some of their property back, but Chernyshyov managed to evade the trap and returned to Russian base in Włodawa October 18.
In November, Chichagov, based in Slonim, discovered imminent danger from an Austrian pursuit corps, and dispatched Chernyshyov to rear guard.
Next day, Chichagov ordered Chernyshyov to march through the enemy territory and make contact with Russian corps of general Peter Wittgenstein.
Accession of Nicholas to the throne and the aftermath of Decembrist revolt accelerated Chernyshyov's career, who joined the close circle of tsar's most trusted officers.
Ageing Chernyshyov was sent to honorary retirement upon ascension of Alexander II of Russia, which coincided with the final stage of the disastrous Crimean War.