The core of the county, named after the river Haine, stretched southeast to include the Avesnois region and southwest to the Selle (Scheldt tributary).

Hainaut appears in 8th-century records as a Frankish gau or pagus which included the Roman towns of Famars and Bavay.

Examples of such personal unions include the following: In 1432, Hainaut, Holland and Zeeland joined Flanders, Artois, Namur, Brabant, Limbourg, and later Luxembourg, within the large agglomeration of territories in the Low Countries belonging to the French House of Valois-Burgundy.

[2] The geographical definition of Hainaut as found in the oldest medieval records, was relatively stable, as shown by Faider-Feytman, Deru, and other historians of the region.

According to archaeological evidence, geographical Hainaut, including Avesnes, formed the oldest region of development in the civitas of the Belgic Nervii.

The related word "gau", used in the modern Dutch and German names of Hainaut, Henegouwen and Hennegau, was also used but never became popular in medieval documents concerning this particular area.

Most of the early medieval records mentioning Hainaut, starting in the 9th century, describe it as a pagus, a land or country, rather than a county.

8th- and 9th-century attestations, as listed by Ulrich Nonn, however, never name any specific counts who ruled it:[5] Many such early medieval pagi in Europe have histories going back to the Roman Empire.

The region is associated by many historians such as Leon Vanderkindere with the so-called Reginarid dynasty who were a powerful and rebellious Lotharingian family, known for their frequent use of the name Reginar.

However, while the later family clearly claimed to have once had important rights throughout Lotharingia, the exact nature of most of these is unclear, and their possession of a county in Hainaut before Reginar III can not be proven.

[6] The only medieval record which claims that Reginar I had direct lordship over Hainaut was the much later Dudo of Saint-Quentin, who is considered to be unreliable for this period.

[11] The second or third count of Hainaut to be named in a contemporary record was however called Godefrid, starting in 958, the year of Reginar III's exile.

Also in 973, Counts named Amelric and Richizo appeared in a royal grant in favour of Crispin Abbey in Hainaut.

He was succeeded by his son Arnulf III, who was killed at the Battle of Cassel in 1071 in an inheritance dispute with his uncle, Robert I the Frisian.

The victorious Robert acquired Flanders, but his sister-in-law Richilde retained the adjacent Lower Lorraine territories in the Holy Roman Empire as her dowry.

Henry IV ordered the Prince-Bishop of Liège to purchase the fiefs and then return them as a unified county to the countess Richilde and, through the chain of feudal authority, to the Dukes of Lower Lorraine.

In the next generation, Namur was given to a different son than Flanders and Hainaut, which remained together under Baldwin VI/IX, who became the first emperor of the Latin Empire of Constantinople.

After his grandson William died in 1347, these same lordships went to his sister, and were held by members of the Wittelsbach dynasty who also possessed the Dukedom of Bavaria-Straubing.

After the death of Duke William II of Bavaria-Straubing in 1417, Hainaut was inherited by his daughter Jacqueline, who had a powerful opponent in her cousin, Philip the Good.

Charles the Bold of Burgundy, the son of Philip, was however killed at the Battle of Nancy in 1477, and the male line of the Burgundian dukes became extinct.

King Louis XI of France had hoped to take advantage of the death of his cousin, Charles and sent an army to invade the Netherlands.

The southern area, around the towns of Valenciennes, Le Quesnoy and Avesnes, was ceded to France under King Louis XIV.

In 1797, during the French Revolution, the northern part of the county was ceded to France by Emperor Francis II, who was also count of Hainaut.