He was born Charles Maximilian, third son of King Henry II of France and Catherine de' Medici in the royal chateau of Saint - Germain - en - Laye. He was immediately made Duke of Orléans upon his birth, succeeding his older brother Louis, his father's second son who had died in infancy the year before.

He visited England and on 14 May was made a Knight of the Order of the Garter at St George's, Windsor, along with Francis Russell, 2nd Earl of Bedford and Sir Henry Sidney.

His father died in 1559, followed in December 1560 by his elder brother, King Francis II (1544 – 1560). The ten-year-old Charles was immediately proclaimed King and, on 15 May 1561, consecrated as King of France in the cathedral at Reims. Government was dominated by his mother, Catherine de' Medici, who at first acted as regent for her young son. Antoine of Bourbon, himself in line to the French throne and husband to Queen Joan III of Navarre, was appointed lieutenant general of France.

Charles' reign was dominated by the Wars of Religion, which pitted various factions against each other. The Huguenots, the French adherents of Calvinism, had a considerable following among the nobility, while their enemies, later organised into the Catholic League were led by the House of Guise, a cadet branch of the House of Lorraine. Queen Catherine, though nominally a Catholic, tried to steer a middle course between the two factions, attempting to keep (or restore) the peace and augment royal power.

The factions had engaged in violence even before Charles' accession: a group of Huguenot nobles at Amboise had tried to abduct King Francis II and arrest the Catholic leaders - Francis, Duke of Guise and his brother, the Cardinal of Lorraine, followed by cases of Protestant iconoclasm and Catholic reprisals. The regent tried to foster reconciliation at the colloquy at Poissy and, after that failed, made several concessions to the Huguenots in the Edict of Saint - Germain in January 1562, but war began when some retainers of the House of Guise - hoping to avenge the attempt of Amboise - attacked and killed several Huguenot worshippers at Vassy.

Louis of Bourbon, Prince of Conde, brother to the lieutenant - general and the suspected architect of the Amboise conspiracy, had already prepared for war and, taking Vassy as the occasion, assumed the role of a protector of Protestantism and began to seize and garrison strategic towns along the Loire. In return, the monarchy revoked the concessions given to the Huguenots. After the military leaders of both sides were either killed or captured in the battles at Rouen, Dreux and Orléans, the regent mediated a truce and issued the Edict of Amboise (1563).

The war was followed by four years of an uneasy "armed peace", during which Catherine tried to unite the factions in the successful effort to recapture Le Havre from the English. After this victory, Charles declared his legal majority in August 1563, formally ending the regency. However, Catherine would continue to play a principal role in politics and often dominate her son. In March of 1564, the King and his mother set out from Fontainebleau on a tour of the war torn kingdom. Their tour spanned two years and brought them through Bar, Lyon, Salon — where they visited Nostradamus, Carcassonne, Toulouse - where the King and his younger brother Henry were confirmed —, Bayonne, La Rochelle and Moulins. During this trip, Charles IX issued the Edict of Roussillon, which standardised 1 January as the first day of the year throughout France.

War again broke out in 1567 after reports of iconoclasm in Flanders prompted Charles to support Catholics there. Huguenots, fearing a Catholic attack was imminent, tried to abduct the King at Meaux, seized various cities and massacred Catholics at Nîmes. After the Battle of Saint Denis saw both a Huguenot defeat and the death of the royal commander-in-chief, the short war ended in 1568 with another truce. However, the significant privileges granted to Protestants were widely opposed, leading to their cancellation and the resumption of war, in which the Dutch Republic, England and Navarra intervened on the Protestant side, while Spain, Tuscany and the Pope supporting the Catholics. Finally, the royal debt and the King's desire to seek a peaceful solution in August 1570 led to yet another truce, which again gave concessions to the Huguenots.

On 26 November 1570 Charles married Elisabeth of Austria, with whom he fathered one daughter, Marie Elisabeth of Valois (1572 – 1578). In 1573, Charles fathered an illegitimate son, Charles, Duke of Angoulême, with his mistress, Marie Touchet.

After the Peace of Saint Germain in 1570, the King increasingly came under the influence of Admiral Gaspard de Coligny, who during the war had succeeded the slain Prince of Condé as leader of Huguenots. The Queen Mother, however, became increasingly fearful of Coligny's unchecked power, especially since the Admiral was pursuing an alliance with England and the Dutch. Coligny was also hated by Henry, Duke of Guise, who accused the Admiral of having ordered the assassination of his father during the siege of Orléans in 1562.

During the peace settlement, a marriage had been arranged between Charles' sister Margaret (1553 – 1615) and Henry de Bourbon, heir to the throne of Navarre, and one of the leading Huguenots. Many Huguenot nobles, including Admiral de Coligny, thronged into Paris for the wedding, which was set for 18 August 1572. On August 22, a failed attempt on Coligny's life — the originator of which remains unclear — put the city in a state of apprehension, as both visiting Huguenots and Parisian Catholics feared an attack by the other side.

In this situation, in the early morning of 24 August 1572, the Duke of Guise moved to avenge his father and murdered Coligny in his lodgings. As Coligny's body was thrown into the street, Parisians mutilated the body and then erupted into a full scale massacre of Huguenots, which was to last five days. Henry of Navarre managed to avoid death by pledging to convert to Catholicism. Over the next few weeks the disorder spread to more cities across France. In total, up to 10,000 Huguenots were killed in Paris and the provinces.

Though the massacres severely weakened Huguenot power, it also reignited war, which ceased after the Edict of Boulogne in 1573 granted Huguenots amnesty and limited religious freedom. However, 1574 saw a failed Huguenot coup at Saint Germain and successful Huguenot uprisings in Normandy, Poitou and the Rhône valley, setting the stage for another round of war.

Having witnessed the horrors of a massacre he had neither approved of nor predicted, the King's fragile mental and physical constitution drastically weakened. His moods swung from boasting about the extremity of the massacre to exclamations that the screams of the murdered Huguenots kept ringing in his ears. Frantically, he blamed alternately himself - "What blood shed! What murders! he cried to his nurse. What evil council I have followed! O my God, forgive me... I am lost! I am lost!" - or his mother - "Who but you is the cause of all of this? God's blood, you are the cause of it all!" The Queen mother responded by declaring she had a lunatic for a son. His physical condition, tending towards tuberculosis, deteriorated to the point where, by spring of 1574, the hoarse coughing turned bloody and the hemorrhages grew more violent.

On his last day, 30 May, Charles called for Henry of Navarre, embraced him, and said, "Brother, you are losing a good friend. Had I believed all that I was told, you would not be alive. But I always loved you... I trust you alone to look after my wife and daughter. Pray God for me. Farewell."

Charles IX died on 30 May at the Château de Vincennes, aged twenty - four years. As his younger brother, Henry, Duke of Anjou had recently been elected King of Poland and was away from France, their mother Catherine resumed the regency until Henry's return from Poland.

Charles had an interest in hunting, and he wrote a book on the subject, La Chasse Royale, which was published long after his death, in 1625. It is a valuable source for those interested in the history of hounds and hunting.

Charles IX is a supporting character in Alexandre Dumas' historical novel Queen Margot, which focuses on the marriage between Henry of Navarre and Margaret of Valois. The book depicts Charles as a frail and sickly ruler, who is complicit in the massacres engineered by his mother and dies after reading a book poisoned with arsenic, which his mother intended for Henry of Navarre.