Francis II became king at the age of fifteen.
In what has been called a coup d'état, the Cardinal of Lorraine and the Duke of Guise—whose niece, Mary, Queen of Scots, had married Francis II the year before—seized power the day after Henry II's death and quickly moved themselves into the Louvre Palace with the young couple.
The English ambassador reported a few days later that "the house of Guise ruleth and doth all about the French king".
For the moment, Catherine worked with the Guises out of necessity.
She was not strictly entitled to a role in Francis's government, because he was deemed old enough to rule for himself.
Nevertheless, all his official acts began with the words: "This being the good pleasure of the Queen, my lady-mother, and I also approving of every opinion that she holdeth, am content and command that ...".
Catherine did not hesitate to exploit her new authority.
One of her first acts was to force Diane de Poitiers to hand over the crown jewels and return the Château de Chenonceau to the crown.
She later did her best to efface or outdo Diane's building work there.

The Guise brothers set about persecuting the Protestants with zeal.
Catherine adopted a moderate stance and spoke against the Guise persecutions, though she had no particular sympathy for the Huguenots, whose beliefs she never shared.
The Protestants looked for leadership first to Antoine de Bourbon, King of Navarre, the First Prince of the Blood, and then, with more success, to his brother, Louis de Bourbon, Prince of Condé, who backed a plot to overthrow the Guises by force.
When the Guises heard of the plot, they moved the court to the fortified Château of Amboise.
The Duke of Guise launched an attack into the woods around the château.
His troops surprised the rebels and killed many of them on the spot, including the commander, La Renaudie.
Others they drowned in the river or strung up around the battlements while Catherine and the court watched.

In June 1560, Michel de l'Hôpital was appointed Chancellor of France.
He sought the support of France's constitutional bodies and worked closely with Catherine to defend the law in the face of the growing anarchy.
Neither saw the need to punish Protestants who worshipped in private and did not take up arms.
On 20 August 1560, Catherine and the chancellor advocated this policy to an assembly of notables at Fontainebleau.
Historians regard the occasion as an early example of Catherine's statesmanship.
Meanwhile, Condé raised an army and in autumn 1560 began attacking towns in the south.
Catherine ordered him to court and had him imprisoned as soon as he arrived.
He was tried in November, found guilty of offences against the crown, and sentenced to death.
His life was saved by the illness and death of the king, as a result of an infection or an abscess in his ear.

When Catherine had realized Francis was going to die, she made a pact with Antoine de Bourbon by which he would renounce his right to the regency of the future king, Charles IX, in return for the release of his brother Condé.
As a result, when Francis died on 5 December 1560, the Privy Council appointed Catherine as governor of France (gouvernante de France), with sweeping powers.
She wrote to her daughter Elisabeth: "My principal aim is to have the honour of God before my eyes in all things and to preserve my authority, not for myself, but for the conservation of this kingdom and for the good of all your brothers".
Francis II became king at the age of fifteen. In what has been called a coup d'état, the Cardinal of Lorraine and the Duke of Guise—whose niece, Mary, Queen of Scots, had married Francis II the year before—seized power the day after Henry II's death and quickly moved themselves into the Louvre Palace with the young couple. The English ambassador reported a few days later that "the house of Guise ruleth and doth all about the French king". For the moment, Catherine worked with the Guises out of necessity. She was not strictly entitled to a role in Francis's government, because he was deemed old enough to rule for himself. Nevertheless, all his official acts began with the words: "This being the good pleasure of the Queen, my lady-mother, and I also approving of every opinion that she holdeth, am content and command that ...". Catherine did not hesitate to exploit her new authority. One of her first acts was to force Diane de Poitiers to hand over the crown jewels and return the Château de Chenonceau to the crown. She later did her best to efface or outdo Diane's building work there.
The Guise brothers set about persecuting the Protestants with zeal. Catherine adopted a moderate stance and spoke against the Guise persecutions, though she had no particular sympathy for the Huguenots, whose beliefs she never shared. The Protestants looked for leadership first to Antoine de Bourbon, King of Navarre, the First Prince of the Blood, and then, with more success, to his brother, Louis de Bourbon, Prince of Condé, who backed a plot to overthrow the Guises by force. When the Guises heard of the plot, they moved the court to the fortified Château of Amboise. The Duke of Guise launched an attack into the woods around the château. His troops surprised the rebels and killed many of them on the spot, including the commander, La Renaudie. Others they drowned in the river or strung up around the battlements while Catherine and the court watched.
In June 1560, Michel de l'Hôpital was appointed Chancellor of France. He sought the support of France's constitutional bodies and worked closely with Catherine to defend the law in the face of the growing anarchy. Neither saw the need to punish Protestants who worshipped in private and did not take up arms. On 20 August 1560, Catherine and the chancellor advocated this policy to an assembly of notables at Fontainebleau. Historians regard the occasion as an early example of Catherine's statesmanship. Meanwhile, Condé raised an army and in autumn 1560 began attacking towns in the south. Catherine ordered him to court and had him imprisoned as soon as he arrived. He was tried in November, found guilty of offences against the crown, and sentenced to death. His life was saved by the illness and death of the king, as a result of an infection or an abscess in his ear.
When Catherine had realized Francis was going to die, she made a pact with Antoine de Bourbon by which he would renounce his right to the regency of the future king, Charles IX, in return for the release of his brother Condé. As a result, when Francis died on 5 December 1560, the Privy Council appointed Catherine as governor of France (gouvernante de France), with sweeping powers. She wrote to her daughter Elisabeth: "My principal aim is to have the honour of God before my eyes in all things and to preserve my authority, not for myself, but for the conservation of this kingdom and for the good of all your brothers".
Catherine de' Medici