Cosimo came to power at 17, when Duke Alessandro de' Medici was assassinated in 1537, as Alessandro's only male issue was illegitimate. He was from a different branch of the family, and so far had lived in Mugello, being almost unknown in Florence: however, many of the influential men in the city favored him, in some cases perhaps hoping to rule through him, taking advantage of his age. However, as Benedetto Varchi famously put it "One bill had the glutton in mind, and another the innkeeper" . 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.
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 last of the Medici Grand Dukes, Gian Gastone de' Medici (1671–1737).
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 he constituted the Grand Duchy of Tuscany.
Cosimo was a despotic ruler and also found it necessary to lay 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. He was also 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). The financial help granted to Charles V allowed him to free Tuscany from the Spanish-Imperial garrisons, and to increase as much as possible its independence from the overwhelming Spanish influence in Italy.
Cosimo was an authoritarian ruler: in order to counter plots, he created for himself a Guard of Swiss mercenaries. In 1548 he managed to have his relative Lorenzino, the last Medici claimant to Florence, assassinated in Venice.
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 at Castello, outside Florence.
Among his many accomplishments was the creation of the Uffizi, originally intended to house the government, now one of the world's great art galleries. He also finished the Pitti Palace as a home for the Medici and created the magnificent Boboli Gardens behind the Pitti. He was a great patron of the arts, supporting, among others, Vasari, Cellini, Pontormo, Bronzino, the architect Lanci, and the historians Scipione Ammirato and Benedetto Varchi.
In 1539, he married Eleonora di Toledo (1522–1562), the daughter of Don Pedro Álvarez de Toledo, the Spanish viceroy of Naples. Her married name was Leonor de Toledo-de' Medici (at right). 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 della Signoria 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.
Cosimo's children were:
|Maria||April 3, 1540 – November 19, 1557|
|Francesco||March 25, 1541 – October 19, 1587|
|Isabella||August 31, 1542 – July 16, 1576||Was murdered by her husband Paolo Giordano I Orsini because of her infidelity.|
|Giovanni||September 28, 1543 – November 1562||Bishop of Pisa and cardinal|
|Lucrezia||June 7, 1545 – April 21, 1561||Wife (1560) of Alfonso II d'Este, Duke of Ferrara and Modena|
|Pietro (Pedricco)||August 10, 1546 – June 10, 1547|
|Garzia||July 5, 1547 – December 12, 1562|
|Antonio||1548 – 1548|
|Ferdinando||July 30, 1549 – February 17, 1609||Grand Duke of Tuscany|
|Anna||1553 – 1553|
|Don Pietro de' Medici||June 3, 1554 – April 25, 1604||Murdered his wife because of her infidelity|
In 1570, he married a second time to Camilla Martelli (died 1574). They had 1 child:
|Virginia||May 29, 1568 – January 15, 1615||Wife of Cesare d'Este, Duke of Modena|
He had two children by his mistress Eleonora degli Albrizzi; Giovanni was legitimized by his father.
|Giovanni||1563 – 1621|
|Unnamed daughter||1566 – 1566||Died before being baptised|
He also had an illegitimate daughter, Bia de' Medici, born before his first marriage.