Franz I (December 8, 1708 – August 18, 1765) was Holy Roman Emperor and Grand Duke of Tuscany, though his wife Maria Theresa effectively executed the real powers of those positions. They were the founders of the Habsburg-Lorraine dynasty. From 1728 until 1737 he was Duke of Lorraine.
Franz-Stephen was born in Nancy, Lorraine (now in France), the oldest surviving son of Leopold, Duke of Lorraine, and his wife Princess Élisabeth Charlotte d’Orléans, the daughter of Philippe I, Duke of Orléans, (brother of King Louis XIV of France and Navarre) and of his second wife Elizabeth Charlotte of the Palatinate, daughter of Carl I Ludwig, Elector Palatine of the Simmern branch of the House of Wittelsbach, and Landgravine Charlotte of Hesse-Cassel. Franz-Stephen was connected by blood to the with the Habsburgs through his grandmother Eleonore, who was the daughter of Holy Roman Emperor Ferdinand III. He was very close to his brother Carl-Alexander and sister Anne Charlotte.

Franz I Stephen, Holy Roman Emperor and Grand Duke of Tuscany
Holy Roman Emperor Carl VI favoured the family, who, besides being his cousins, had served the House of Habsburg with distinction. He had designed to marry his daughter Maria Theresa to Franz-Stephen’s older brother Leopold Clement. On Leopold Clement’s death, Emperor Karl adopted the younger brother as his future son-in-law. Franz-Stephen was brought up in Vienna with Maria Theresa with the understanding that they were to be married, and a real affection arose between them.
When the War of the Polish Succession broke out in 1733, France used it as an opportunity to seize Lorraine, since France’s prime minister, Cardinal Fleury, was concerned that, as a Habsburg possession, it would bring Austrian power too close to France.
A preliminary peace was concluded in October 1735 and ratified in the Treaty of Vienna in November 1738. Under its terms, Stanisław I, the father-in-law of King Louis XV of France and the losing claimant to the Polish throne, received Lorraine, while Franz-Stephen, in compensation for his loss, was made heir to the Grand Duchy of Tuscany, which he would inherit in 1737.
On January 31, 1736 Franz-Stephen agreed to marry Maria Theresa. He hesitated three times (and laid down the feather before signing). Especially his mother Élisabeth Charlotte d’Orléans and his brother Prince Carl-Alexander of Lorraine were against the loss of Lorraine. On February 1, Maria Theresa sent Franz-Stephen a letter: she would withdraw from her future reign, when a male successor for her father appeared.
They married on February 12, 1736, in the Augustinian Church, Vienna. The (secret) treaty between the Emperor Carl VI and Franz-Stephen was signed on May 4, 1736. In January 1737, the Spanish troops withdrew from Tuscany, and were replaced by 6,000 Austrians. On January 24, 1737 Franz-Stephen received the Grand Duchy of Tuscany from his father-in-law. Until then, Maria Theresa was Duchess of Lorraine. In 1744 Franz-Stephen’s brother Carl-Alexander married the younger sister of Maria Theresa, Archduchess Maria Anna of Austria. In 1744 Carl became governor of the Austrian Netherlands, a post he held until his death in 1780.

Carl VII Albert of Bavaria, Holy Roman Emperor
As son-in-law of Joseph I, Holy Roman Emperor, Carl-Albert of Bavaria rejected the Pragmatic Sanction of 1713 and claimed the German territories of the Habsburg dynasty after the death of emperor Carl VI in 1740. After the two year War of the Austrian Succession he was elected as Holy Roman Emperor Carl VII from January 24, 1742 until his death in 1745. As a member of the House of Wittelsbach, Carl VII was the first person not born of the House of Habsburg to become emperor in three centuries, though he was connected to that house both by blood and by marriage. Holy Roman Emperor Ferdinand II was his great-great grandfather.
Since a woman could not be elected Holy Roman Empress, Maria Theresa wanted to secure the imperial office for her husband, but Franz-Stephen did not possess enough land or rank within the Holy Roman Empire. In order to make him eligible for the imperial throne and to enable him to vote in the imperial elections as elector of Bohemia (which she could not do because of her sex), Maria Theresa made Franz-Stephen co-ruler of the Austrian and Bohemian lands on November 21, 1740.

Empress Maria Theresa, Queen of Hungary and Bohemia, Archduchess of Austria, Empress Consort of the Holy Roman Empire
It took more than a year for the Diet of Hungary to accept Franz-Stephen as co-ruler, since they asserted that the sovereignty of Hungary could not be shared. Despite her love for him and his position as co-ruler, Maria Theresa never allowed her husband to decide matters of state and often dismissed him from council meetings when they disagreed.
The Treaty of Breslau of June 1742 ended hostilities between Austria and Prussia. With the First Silesian War at an end, the Queen Maria Theresa soon made the recovery of Bohemia her priority. French troops fled Bohemia in the winter of the same year. On May 12, 1743, Maria Theresa had herself crowned Queen of Bohemia in St. Vitus Cathedral.
Prussia became anxious at Austrian advances on the Rhine frontier, and Friedrich II of Prussia again invaded Bohemia, beginning a Second Silesian War; Prussian troops sacked Prague in August 1744. The French plans for the war fell apart when Holy Roman Emperor Carl VII Albert died in January 1745.
Franz-Stephen was elected Holy Roman Emperor on September 13, 1745 as Franz I. Prussia recognised Francis as emperor, and Maria Theresa once again recognised the loss of Silesia by the Treaty of Dresden in December 1745, ending the Second Silesian War.
Although Franz was the Holy Roman Emperor, his wife Maria Theresa was the sovereign in her own right in the Habsburg hereditary lands of Austria, Hungary, Croatia, Bohemia, Transylvania, Mantua, Milan, Lodomeria and Galicia, the Austrian Netherlands, and Parma. By marriage, she was Duchess of Lorraine, Grand Duchess of Tuscany and Holy Roman Empress.
Franz was well content to leave the wielding of power to his able wife. He had a natural fund of good sense and brilliant business capacity and was a useful assistant to Maria Theresa in the laborious task of governing the complicated Austrian dominions, but he was not active in politics or diplomacy. However, his wife left him in charge of the financial affairs, which he managed well until his death. Heavily indebted and on the verge of bankruptcy at the end of the Seven Years’ War, the Austrian Empire was in a better financial condition than France or England in the 1780s. He also took a great interest in the natural sciences. He was a member of the Freemasons.
Franz was a serial adulterer, many of his affairs well-known and indiscreet, notably one with Maria Wilhelmina, Princess of Auersperg, who was thirty years his junior. This particular affair was remarked upon in the letters and journals of visitors to the court and in those of his children.
Franz died suddenly in his carriage while returning from the opera at Innsbruck on 18 August 1765. He is buried in tomb number 55 in the Imperial Crypt in Vienna.
Maria Theresa and Franz I had sixteen children, amongst them the last pre-revolutionary queen consort of France, their youngest daughter, Marie Antoinette (1755–1793), who married King Louis XVI of France and Navarre. Franz was succeeded as Holy Roman Emperor by his eldest son, Joseph II, and as Grand Duke of Tuscany by his younger son, Peter Leopold (later Emperor Leopold II). Maria Theresa retained the government of her dominions as their sovereign until her own death in 1780.