Pyotr was born in 1765 to a prince of the Mukhrani branch of the Bagrationi dynasty, Colonel Prince Ivane Bagrationi, who was the eldest son of Prince Alexander, an illegitimate son of King Jesse of Kartli, which is now central Georgia. He studied Russian and German and was taught Persian, Turkish, Armenian, and Georgian by his father. However, unlike many other Russian aristocrats, he did not know French. Bagration personally identified himself as a "pure Russian" (chistoi russkoi).
Pyotr joined the Imperial Russian Army in 1782, enlisting as a sergeant in the Kavsansk Rifles of the Astrakhan Infantry Regiment. His younger brother Roman joined the Chuguevsk Cossack regiment as a uryadnik (a Cossack NCO) at the age of thirteen in 1791. Both would go on to become generals of the Imperial Russian Army.
Bagration served for some years in the Russian-Circassian War. He participated in the Siege of Ochakov (1788). In 1792 he was commissioned as a Captain and transferred to the Kiev Cavalry Regiment that year as a second Major, transferring as a full first Major to the Sofiiskii Carabineers on 15 May 1794. He served in the military campaign to suppress the Polish Kościuszko Uprising of 1794.
He received successive promotions to Lieutenant-Colonel (26 October 1794), to Colonel (1798) and to Major-General (1799). His merits were recognized by Suvorov, whom he accompanied in the Italian and Swiss campaigns of 1799, winning particular distinction by the capture of the town of Brescia. From 1798 to 1799, he commanded the 6th Chasseurs; from 1801 to 1802, he commanded the Chasseurs of the Imperial Guard; then from 1802 to 1805, he served as GOC Jager Brigade.
He was the alleged lover of Emperor Paul's daughter Catherine. In 1800 Paul recognized the title of "Prince (Knyaz) Bagration" for Pyotr in Russia, and unexpectedly married him off to Countess Catherine Pavlovna Skavronskaya, the favorite niece of Grigory Potemkin and one of the Empress Maria's ladies-in-waiting. Bagration and Catherine had been casually involved, but the marriage was a failure. The young and lovely Catherine soon preferred traveling and, in 1805, fled to Vienna, where her salon and running affair with Prince Clemens von Metternich—who called her "the Naked Angel"—permitted her to serve as an important agent of Russian intelligence and diplomacy. Bagration was obliged by the emperor to claim their daughter, Marie-Clementine, as his own and to subsidize thousands of rubles of Catherine's debts. He had a reputation as a heavy gambler, as well, and was forced to sell estates to cover losses that rose as high as 80,000 roubles.
In the wars of 1805 Bagration's achievements appeared even more brilliant. When Napoleon ordered Murat to break an armistice he had just signed with Bagration, the general was able to successfully resist the repeated attacks of forces five times his own numbers under Murat and Lannes at Schöngrabern (16 November) near Hollabrunn. Though Bagration lost half of the men under his command, their stand protected the retreat of the main army under Kutuzov to Olmutz. When Kutuzov was overruled and forced into battle at Austerlitz (2 December), Bagration commanded the advance guard of the Prince Liechtenstein's column and defended the allied right against Lannes while the left attacked Napoleon's deliberately undefended right flank. He was promoted to Lieutenant-General in 1805, and in 1807 fought bravely and obstinately at the battles of Eylau (7 February), Heilsberg (11 June), and Friedland (14 June).
He was successful as commander of both Russia's Finnish Campaign in 1808 and Turkish Campaign in 1809. In the former, he captured the Åland Islands by a daring march across the frozen Gulf of Finland. His rapid transfer to the distant Moldavian front against the Ottoman Empire has been seen as a reprimand for an alleged affair with the tsarevna Catherine, who was married off shortly thereafter. While there, he led the Russian army at Rassowa and Tataritza and was promoted to full General of Infantry.
In 1812, Bagration commanded the 2nd Army of the West. A few days before Napoleon's invasion on 24 June, he suggested to Alexander I a pre-emptive strike into the Duchy of Warsaw. Defeated at Mogilev (23 July), Bagration led his forces to join the 1st Army at Smolensk under Barclay de Tolly, to whom he ceded overall command of both armies on 2 August. Bagration led the left wing at the Battle of Borodino (7 September) where he constructed a number of flèches which, due to a shortage of engineer officers, were poorly-built. During the battle he received a mortal wound and later died on 24 September, in the village of Simi, which belonged to his aunt.
It is said that, while wounded, Bagration kept giving orders to the troops without knowing that the Russian army was abandoning Moscow. When he finally heard the truth, Bagration was so shocked that he rapidly stood up, totally forgetting about his grave wound. Such an act was too much for his severely wounded body and it quickly cost Bagration his life.