The city of Alençon was probably founded in the fourth century by Alan soldiers. The name is first seen in a document dated in the seventh century. During the tenth century, Alençon was a buffer state between Normandy and the Maine regions. In 1049-1051, William Duke of Normandy, later known as William the Conqueror and king of England, laid siege to the town, which had risen in support of the Count of Anjou along with two other towns of the Bellême estates, Domfront (then in Maine) and Bellême (held directly from King Henry I of France). According to Duke William's chaplain and panegyrist, William of Poitiers, the citizens insulted William by hanging animal skins from the walls, in reference to his ancestry as the illegitimate son of Duke Robert and a tanner's daughter. On capturing the town, William had a number of the citizens' hands and feet cut off in revenge. Alençon was occupied by the English during the Anglo-Norman wars of 1113 to 1203.
The city became the seat of a dukedom in 1415, belonging to the sons of the King of France until the French Revolution, and some of them played important roles in French history: see Duke of Alençon. The French Revolution caused relatively little disorder in this area although there were some royalist uprisings nearby.
A long-standing local fabric industry gave birth to the town's famous point d'Alençon lace in the 18th century. The economic development of the nineteenth century was based on iron foundries and mills in the surrounding region. In the first half of the twentieth century the city developed a flourishing printing industry.
Alençon was home to Marie-Azélie Guérin Martin and Louis Martin, the parents of St. Thérèse of Lisieux. They were the first spouses in the history of the Catholic Church to be proposed for sainthood as a couple, in 2008. Zélie and Louis were married at the Basilica of Notre-Dame in Alençon on 12 July 1858 and spent their whole married life in Alençon, where Thérèse was born in January 1873 and spent her early childhood until the death of her mother in 1877. 
After the war the population sharply increased and new industries settled. Many of these were related to plastics and the town is now a major plastics educational centre.