Francesco Stefano di Lorena was Holy Roman Emperor and Grand Duke of Tuscany even though the titles actually belonged to his wife. With his wife, Maria Teresa, he was the founder of the Habsburg-Lorraine dynasty. From 1728 to 1737 it was Duke of Lorraine; he lost this title when he surrendered Lorraine to France.
Francis was born in Nancy in Lorraine (now France) the oldest surviving sons of Leopoldo Duke of Lorraine and his wife Elisabetta Carlotta d'Orléans daughter of Philip I, Duke of Orléans. He was related to the Habsburgs through his grandmother Eleonora Maria daughter of Emperor Ferdinand III of the Holy Roman Empire and wife of Carlo Leopoldo di Lorena, his grandfather.
The emperor Charles VI favored the family, which, besides being his cousins, had served the house of Austria with distinction. He had planned to marry his daughter Maria Teresa with Francesco's older brother, Leopoldo Clemente. On the death of Leopoldo Clemente, Charles adopted his younger brother as his future son-in-law. Francis was educated in Vienna with Maria Teresa, on the agreement that they would get married, and a true affection arose between them.
At the age of 15, when he was taken to Vienna, he was invested with the Duchy of Teschen, which had been mediatized and granted to his father by the emperor in 1722. Francesco Stefano di Lorena succeeded his father as Duke of Lorraine in 1729. Maria Theresa of Austria had established in 1732 that Francis became "Lord Lieutenant" (locumtenens) of Hungary. He was not enthusiastic about this position, but Maria wanted him closer to her. In June 1732 he agreed to go to Presburg.
At the end of the War of Polish Succession, Emperor Charles VI agreed to indemnify the French candidate Stanislao Leszczyński (father-in-law of Louis XV of France) for the loss of his crown in 1735.
Prime Minister of France Cardinal de Fleury saw the Polish struggle as a possibility of striking Austrian power in the West without the assent's opinion. While he cared little for who should become King of Poland, the reason to protect the king's father-in-law was more congenial to him and he hoped to use war as a means to humiliate the Austrians and perhaps guarantee the much desired Duchy of Lorraine by his duke, Francis Stephen, who was supposed to marry the daughter of Emperor Charles, Maria Theresa, who would bring Austrian power dangerously close to the French border.
A preliminary peace was concluded in October 1735 and ratified by the Treaty of Vienna in November 1738. Augustus was confirmed as king of Poland, Stanislaus was indemnified with Lorraine (who at his death, through his daughter, would have passed to France ) while the former Duke of Lorraine was created heir of the Grand Duchy of Tuscany, which he inherited in 1737.
Although the battle was interrupted after the preliminary peace agreement in 1735, the final peace agreement had to wait until the death of the last of the Medici, the Grand Duke of Tuscany, Gian Gastone in 1737, to allow the territorial exchanges foreseen by the plan of peace to enter into force.
In March 1736 the emperor persuaded Francesco, his future son in law (!) [Without source] to exchange Lorraine for the Grand Duchy of Tuscany. France had requested that Maria Teresa's fiancé deliver the Duchy of Lorraine to house the deposed king of Poland. The emperor considered other possibilities (such as making her marry the future Charles III of Spain), before announcing the couple's engagement. If something went wrong, Francesco would become governor of the Austrian Netherlands.
Even Elisabetta di Parma wanted the Grand Duchy of Tuscany for her son Carlo; Grand Duke Gian Gastone was childless and was related to Elizabeth through his great-grandmother Margherita de 'Medici. As a result, Elizabeth's son could claim his right to the throne as a descendant of Margaret.
On January 31st, 1736, Francis had accepted to marry Maria Teresa. He hesitated three times (and fixed the feathers before signing). [Citation needed] Above all his mother Elizabeth Charlotte of Orleans and her brother Prince Charles Alexander of Lorraine were opposed to the loss of Lorraine.
Francis and Maria Theresa of Austria were married on 12 February in the Church of the Augustinians. The marriage took place on February 14, 1736. The treaty (secret) between the Emperor and Francis was signed May 4, 1736. In January 1737, the Spanish troops withdrew from Tuscany, and were replaced by 6,000 Austrians. On January 24, 1737, Francesco received Tuscany from his father-in-law. Until then, Maria Teresa was the Duchess of Lorraine. In order to guarantee the independence of Tuscany and not to make it a region of the Hapsburg state it was decided to keep the two crowns separate, keeping the imperial title for the firstborn of the Habsburg family - Lorena, while for the second-born the grand-ducal.
Gian Gastone de 'Medici, who died on 9 July 1737, was Francesco's second cousin. In June 1737 Francis went to Hungary to fight again against the Turks. In October 1738 he was back in Vienna. On December 17, 1738, the couple traveled south, accompanied by Francesco Stefano's brother, Carlo, in Florence for three months. They arrived on January 20, 1739.  but, having accepted the task against the will, he never resided in the city, preferring the Viennese court and leaving an advice to manage Tuscany.
In 1744 Francesco's brother, Carlo married a younger sister of Maria Teresa, the Archduchess Maria Anna of Austria. In 1744 Charles became governor of the Austrian Netherlands, (until his death in 1780).
Maria Teresa granted him with the Treaty of Füssen his election to the Empire on 13 September 1745, at the death of Charles Albert of Bavaria and made him, moreover, co-regent of her hereditary dominions.
Francesco was happy to let his skilled wife manage power. He had a natural background of common sense and a certain ability in business and was a helpful assistant to Maria Teresa in the laborious task of governing the complicated Austrian domains, but her functions seem to have been above all secretarial. The scarce burdens of the imperial crown, now emptied of royal powers, and the grand-ducal one, left much free time for Francesco Stefano, which he committed by laying the foundations for the imperial family's assets, separated from the state and subject to compulsory tax, which lasted until the requisition after the First World War, by the newly formed Austrian republic. He also had a great interest in the natural sciences and was a member of Freemasonry.
Francesco was quite a womanizer and was known for his very indiscreet relationships, especially that with Princess Auersperg, who was thirty years younger than him. This particular story was observed on letters and magazines of visitors to the judge and his sons.
He died suddenly in his carriage as he returned from the opera house in Innsbruck on 18 August 1765. His wife wrote this in his diary: "The Emperor Francis my wife lived for 56 years, 8 months, 10 days, and died the August 18, 1765 at half past nine in the evening. It therefore lived for 680 months, 2,958 weeks, 20,778 days, 496,992 hours. My happy marriage lasted for 29 years, 6 months and 6 days, or 335 months, 1,540 weeks, 10,781 days and 258,744 hours ». He is buried in tomb number 55 of the Imperial Crypt in Vienna.
Maria Teresa and Francesco I had sixteen children, their youngest daughter was the future queen consort of France, Marie Antoinette (1755-1793). To succeed him officially was the eldest son Joseph II, even really the power remained in the hands of his wife. Another son was the emperor Leopold II.