The first people in Laois were bands of hunters and gatherers who passed through the county about 8,500 years ago. They hunted in the forests that covered Laois and fished in its rivers, gathering nuts and berries to supplement their diets.

Next came Ireland's first farmers. These people of the Neolithic period (4000 to 2500 BC) cleared forests and planted crops. Their burial mounds remain in Clonaslee and Cuffsborough.

By the first century AD, the western third of Laois was part of the Kingdom of Ossory. The eastern part was divided roughly into seven parts, which were ruled by the Seven Septs of Loígis: O’More (O’Moore), O’Lalor, O’Doran, O’Dowling, O’Devoy (O’Deevy), O’Kelly and McEvoy.

The county name of Laois derives from Loígis, of which the modern county is only a part. In the 11th century, its dynastic rulers adopted the surname Ua/Ó Mórdha (O’Moore). They claimed descent from a member of the Red Branch Knights.

The fortress on the Rock of Dunamase was part of the dowry of the Irish princess Aoife, who was given in marriage in 1170 to the Norman warrior Strongbow at the start of the Norman Conquest.

The present day county was formed following the Tudor plantations in an attempt by the English Crown to expand its sphere of influence in Ireland which had been in  decline.

In 1548, the English confiscated the lands of the O’Mores and built "Campa", known as the Fort of Leix, today's Portlaoise. In 1556, the town was named Maryborough and Laois renamed Queen’s County in honor of the English Queen, Mary Tudor.

All the clans from Laois had their lands confiscated after the Cromwellian Invasion in 1648.

The Great Hunger/ Famine of 1845–49 devastated the county. The county's workhouses could not cope with the number of destitute people seeking shelter. By the time the workhouse opened at Donaghmore in 1853, many of the poorest had emigrated or died.

From 1880 to 1881, a Land War convulsed the county, as members of the Land League challenged the authority of the landlords. Evicted tenants and other destitute people filled the county’s workhouses. After the Land Act of 1881, tenants and landlords formed an uneasy truce.

By the foundation of the State in 1922, Celts and Vikings, Gaelic lords and Norman knights, monks and Huguenots, landlords and land leaguers, had all left their mark on this county. The new Ireland gave the county its old name back. Queen’s County was once again County Laois.