Created in 1556 by Mary I of England (Bloody Mary) as Queen's County, Laois (also spelt Laoighis or Leix) received its present Irish language name following the Irish War of Independence. Portlaoise (previously Maryborough) is the county town. The county was renamed in the early 1920s, following a competition.

Laois was the subject of two Plantations or colonisations by English settlers. The first occurred in 1556, when the Earl of Sussex dispossessed the O'More clan from the area and attempted to replace them with English settlers. However, this only led to a long drawn out guerilla war in the county and left a small English community clustered around garrisons. There was a more successful plantation in the county in 17th century, which expanded the existing English settlement with more landowners and tenants from England. Neither plantation was fully successful due to a lack of tenants and because of continuous raids and attacks by the O' Mores.

Finally, the county became home to a community of French Huguenots in the 1690s, who were settled in Ireland after their service to William of Orange in the Williamite war in Ireland. In addition to this, large numbers of Quakers settled in Mountmellick and developed the area.

The county is landlocked and, uniquely amongst Irish counties it does not border any other counties with a sea coast. It is therefore considered to be "the most landlocked county in Ireland."

Located in the heart of the Irish Midlands, County Laois covers an area of 664 square miles. It is a county of great contrasts from the wild hills of the Slieve Bloom Mountains to the gentle landscape of the Central Valley. There are more than 1,000 historical sites in the county, covering a historical period of 8,500 years.