Cosimo I de ' Medici , Grand Duke of Tuscany Cosimo I de ' Medici ( 12 June 1519 – 21 April 1574 ) was the second Duke of Florence from 1537 until 1569 , when he became the first Grand Duke of Tuscany . Life . Rise to power . Cosimo was born in Florence , on June 12 , 1519 , the son of the famous " condottiere " Giovanni dalle Bande Nere from Forlì and Maria Salviati . Cosimo came to power at 17 , when the 26 - year - old Duke , Alessandro de ' Medici , was assassinated in 1537 , as Alessandro ' s only male child was illegitimate . Cosimo was from a different branch of the family , and so far had lived in Mugello , and was almost unknown in Florence : however , many of the influential men in the city favored him , several of them hoping to rule through him and thereby enrich themselves at the state ' s expense . However , as Benedetto Varchi famously put it " The innkeeper ' s reckoning was different from the glutton ' s . " Cosimo proved strong - willed , astute and ambitious , and soon rejected the clause he had signed , which entrusted much of the power to a council of Forty - Eight . When the Florentine exiles heard of the death of Alessandro , they marshalled their forces with support from France and from disgruntled neighbors of Florence . During this time , Cosimo had an illegitimate daughter , Bia ( 1537 – 1542 ) , who was portrayed shortly before her premature death in a marvelous painting by Bronzino . Toward the end of July 1537 , the exiles marched into Tuscany under the leadership of Bernardo Salviati and Piero Strozzi . When Cosimo heard of their approach , he sent his best troops under Alessandro Vitelli to engage the enemy , which they did at Montemurlo , a fortress that belonged to the Nerli . After defeating the exiles ' army , Vitelli stormed the fortress , where Strozzi and a few of his companions had retreated to safety . It fell after only a few hours , and Cosimo celebrated his first victory . The prominent prisoners were subsequently beheaded on the Piazza or in the Bargello . Filippo Strozzi ' s body was found with a bloody sword next to it and a note quoting Virgil , but many believe that his suicide was faked . Rule of Tuscany . In June 1537 Cosimo was recognized as head of the Florentine state by the Holy Roman Emperor Charles V , in exchange for help against France in the course of the Italian Wars . With this move he firmly restored the power of the Medici , who thereafter ruled Florence until the death of the last of the Medici , Gian Gastone de ' Medici , in 1737 . The help granted to Charles V allowed him to free Tuscany from the Imperial garrisons , and to increase as much as possible its independence from the overwhelming Spanish influence in Italy . Cosimo next turned on Siena . With the support of the Emperor , he defeated the Sienese at the Battle of Marciano ( 1554 ) , and laid siege to Siena . Despite the inhabitants ' desperate resistance , on April 17 , 1555 , after a 15 - month siege , the city fell , its population diminished from forty thousand to eight thousand . In 1559 Montalcino , the last redoubt of Sienese independence , was annexed to Cosimo ' s territories . In 1569 , Pope Pius V elevated him to the rank of Grand Duke of Tuscany . In the last 10 years of his reign , struck by the death of two of his sons by malaria , Cosimo gave up the active rule to his son and successor Francesco I de ' Medici , Grand Duke of Tuscany . He retreated to live in his villa , Villa di Castello , outside Florence . Statesmanship . Cosimo was an authoritarian ruler and secured his position by employing a guard of Swiss mercenaries . In 1548 he managed to have his relative Lorenzino , the last Medici claimant to Florence , assassinated in Venice . Cosimo also was an active builder of military structures , in an attempt to save his state from the frequent passage of foreign armies ( examples are the new fortresses of Siena , Arezzo , Sansepolcro , the new walls of Pisa and Fivizzano , and the strongholds of Portoferraio , on the island of Elba , and Terra del Sole ) . He laid heavy tax burdens on his subjects . Despite his economic difficulties , he was a lavish patron of the arts and also developed the Florentine navy , which eventually took part in the Battle of Lepanto , and which he entrusted to his new creation , the military Order of St . Stephen . Patronage of the arts . Cosimo is perhaps best known today for the creation of the Uffizi ( " offices " ) . Originally intended as a means of consolidating his administrative control of the various committees , agencies , and guilds established in Florence ' s Republican past , it now houses one of the world ' s most important collections of art , much of it commissioned and / or owned by various Medici . His gardens at Villa di Castello , designed by Niccolo Tribolo when Cosimo was only seventeen years old , were designed to announce a new golden age for Florence , and to express the magnificence and virtues of the Medici . They were decorated with fountains , a labyrinth , a grotto and ingenious " water jokes , " and were a prototype for the Italian Renaissance garden . They had a profound influence on later Italian and French gardens through the eighteenth century . He also finished the Pitti Palace as a home for the Medici and created the magnificent Boboli Gardens behind the Pitti . As his more prominent ancestors had been , he was also an important patron of the arts , supporting , among others , Vasari , Cellini , Pontormo , Bronzino , the architect Lanci , and the historians Scipione Ammirato and Benedetto Varchi . A large bronze equestrian statue of Cosimo I by Giambologna , erected in 1598 , still stands today in the Piazza della Signoria , the main square of Florence . Cosimo was also an enthusiast of alchemy , a passion he had inherited from his grandmother Caterina Sforza . Marriage and family . In 1539 , he married Eleonora di Toledo ( 1522 – 1562 ) , the daughter of Don Pedro Álvarez de Toledo , the Spanish viceroy of Naples . Her face is still familiar to many because of her solemn and distant portraits by Agnolo Bronzino . The most famous of them , with her son Giovanni , hangs in the Uffizi Gallery . She provided the Medici with the Pitti Palace and seven sons to ensure male succession and four daughters to connect the Medici with noble and ruling houses in Italy . She was a patron of the new Jesuit order , and her private chapel in the Palazzo Vecchio was decorated by Bronzino , who had originally arrived in Florence to provide festive decor for her wedding . She died , with her sons Giovanni and Garzia , in 1562 , when she was only forty ; all three of them were struck down by malaria while traveling to Pisa . Before his first marriage , Cosimo fathered an illegitimate daughter with an unknown woman : With Eleonora , Cosimo fathered eleven children : After Eleonora ' s death in 1562 , Cosimo fathered two children with his mistress Eleonora degli Albrizzi : In 1570 , Cosimo married Camilla Martelli ( died 1590 ) and fathered one child with her :