Montmorency was born at Chantilly, and was brought up with the future King Francis I, whom he followed into Italy in 1515, distinguishing himself especially at Marignano. In 1516 he became governor of Novara; in 1520 he was present at the Field of Cloth of Gold, and afterwards had charge of important negotiations in England. Successful in the defence of Mezieres (1521), and as commander of the Swiss troops in the Italian campaign of the same year, he was made marshal of France in 1522, accompanied Francis into Italy in 1524, and was taken prisoner at Pavia in 1525.
Released soon afterwards, he was one of the negotiators of the treaty of Madrid, and in 1530 reconducted the king's sons into France. On the renewal of the war by Charles V's invasion of France in 1536, Montmorency compelled the emperor to raise the siege of Marseilles; he afterwards accompanied the king of France into Picardy, and on the termination of the Netherlands campaign marched to the relief of Turin. In 1538, on the ratification of the ten years' truce, he was rewarded with the office of constable, but in 1541 he fell into disgrace, and did not return to public life until the accession of Henry II in 1547.
In 1548 he repressed the insurrections in the south-west, particularly at Bordeaux, with great severity, and in 1549-50 conducted the war in the Boulonnais, negotiating the treaty for the surrender of Boulogne on March 24, 1550. In 1551 his barony was erected into a duchy. Soon afterwards his armies found employment in the north-east in connection with the seizure of Metz, Toul and Verdun by the French king. His attempt to relieve St Quentin resulted in his defeat and captivity (August 10, 1557), and he did not regain his liberty until the peace of Cateau-Cambrésis in 1559.
Supplanted in the interval by the Guises, he was treated with coldness by the new king, Francis II, and compelled to give up his mastership of the royal household, his son, however, being appointed marshal by way of indemnity. On the accession of Charles IX in 1560 he resumed his offices and dignities, and, uniting with his former enemies, the Guises, played an important part in the Huguenot war of 1562. Though the arms of his party were victorious at Dreux, he himself fell into the hands of the enemy, and was not liberated until the treaty of Amboise (March 19, 1563). In 1567 he again triumphed at the Battle of Saint-Denis, but received the death-blow of which he died in Paris two days later.