Born at Lemberg in 1677, was the son of Rafael Leszczynski, governor of Poznan Voivodship, and Anne Catherine Jablonowska. He married Catherine Opalinska by whom he had one daughter. In 1697, as cupbearer of Poland, he signed the confirmation of the articles of election of Augustus II. In 1703 he joined the Lithuanian Confederacy, which the Sapiehas with the aid of Swedish gold had formed against Augustus, and in the following year was selected by Charles XII of Sweden to supersede Augustus. Leszczynski was a young man of blameless antecedents, respectable talents, and ancient family, but certainly without sufficient force of character or political influence to sustain himself on so unstable a throne.
Nevertheless, with the assistance of a bribing fund and an army corps the Swedes succeeded in procuring his election by a scratch assembly of half a dozen castellans and a few score of gentlemen (July 2, 1704). A few months later Stanislaw was forced by a sudden inroad of Augustus to seek refuge in the Swedish camp, but finally on September 24, 1705 he was crowned king with great splendour, Charles himself supplying his nominee with a new crown and sceptre in lieu of the ancient regalia which had been carried off to Saxony by Augustus. The first act of the new king was to conclude an alliance with Charles XII whereby Poland engaged to assist Sweden against the tsar. Stanislaw did what he could to assist his patron. Thus he induced Mazeppa the Cossack hetman to desert Peter at the most critical period of the war, and placed a small army corps at the disposal of the Swedes. But he depended so entirely upon the success of Charles's arms that after Poltava (1709) his authority vanished as a dream at the first touch of reality.
The vast majority of the Poles hastened to repudiate him and make their peace with Augustus, and Leszczynski, henceforth a mere pensioner of Charles XII, accompanied Krassau’s army corps in its retreat to Swedish Pomerania. On the restoration of Augustus, Stanislaw resigned the Polish Crown (though he retained the royal title) in exchange for the little principality of Zweibrücken. In 1716 he was saved from assassination at the hands of a Saxon officer, Lacroix, by Stanislaw Poniatowski, the father of the future king. He now resided at Weissenburg in Lorraine, and in 1725 had the satisfaction of seeing his daughter Mary become the consort of Louis XV and queen of France. From 1725 to 1733, Stanislaw lived at Chateau Chambord.
His son-in-law supported his claims to the Polish throne after the death of Augustus II in 1733, which led to the War of the Polish Succession. On the oth of September 1733 Stanislaw himself arrived at Warsaw, having travelled night and day through central Europe disguised as a coachman, and on the following day, despite many protests, was duly elected king of Poland for the second time. But Russia, opposed to any nominee of France and Sweden, at once protested against his election; declared in favour of the new elector of Saxony, at being the candidate of her Austrian ally; and on June 30, 1734 a Russian army of 20,000 under Peter Lacey, after proclaiming Augustus III at Warsaw, proceeded to besiege Stanislaw in Danzig where he had intrenched himself with his partisans (including the primate and the French and Swedish ministers) to await the promised succour from France. The siege began in October i734. On March 17, 1735 Marshal Mllnnich superseded Lacey, and on the May 20, the long expected French fleet appeared in the roads and disembarked 2400 men. A week after its arrival this little army gallantly attempted to force the Russian intrenchments, but was beaten off and finally compelled to surrender. This, by the way, was the first time France and Russia met as foes in the field. On June 30, 1735 Danzig capitulated unconditionally, after sustaining a siege of 135 days which cost the Russians 8000 men. Stanislaw, disguised as a peasant, had contrived to escape two days before. He was first heard of again at Königsberg, whence he issued a manifesto to his partisans which resulted in the formation of a confederation on his behalf, and the despatch of a Polish envoy to Paris to urge France to invade Saxony with at least 40,000 men. In the Ukraine too, Count Nicholas Potocki kept on foot to support Stanislaw a motley host of 50,000 men, which was ultimately scattered by the Russians. On January 26, 1736 Stanislaw again abdicated the throne, but received by way of compensation the dukedom of Lorraine and Bar, which was to revert to France on his death. He settled at Lunéville, founded there the Academia Stanjslai~ and devoted himself for the rest of his life to science and philanthropy.
He died in 1766 at the age of 89. Among his works may be mentioned: OEuvres du philosophe bienfaisant (Paris, 1763; 1866).
See also: List of Polish rulers