One of the earliest known settlements in County Offaly is at Boora Bog which dates to the Mesolithic era. Excavations there provide evidence of a temporary settlement. Stone axes, arrowheads and blades were discovered which date to between 6,800 – 6,000 BCE.

The Kingdom of Uí Failghe from which the name Offaly is derived, was ruled by the Ó Conchobhair Failghe (anglicised as: O'Conor Faly) whose territory included the east of the county. The Kingdom of Firceall ruled by the O'Molloy clan constituted much of the center of the county. Much of the south of the present-day county was ruled by Ó Cearbhaill of Éile (anglicised as  O'Carroll Ely).

Uí Failghe was a petty kingdoms in Gaelic Ireland located just outside the English controlled area known as the Pale. When Mary Tudor succeeded to the English throne in 1553 she set about re-establishing English control of Ireland by ‘planting’ English settlers upon Irish land which she intended to seize from disloyal subjects. The Irish in Offaly were given the option of becoming loyal citizens or be forced off their lands. Whatever land that was vacated or forcibly taken from the native Irish would then be offered to any loyal Englishmen that wanted to move to the Irish midlands and live there. The area now called Offaly was designated Kings County in honor of Queen Mary’s husband King Philip 11 of Spain.

Queen Mary’s plantation was unsuccessful but in the 17thcentury all the Offaly clans had their lands confiscated after the Cromwellian Invasion. Some O'Carroll's immigrated to the Americas in the 18thcentury.  Charles Carroll is the only Catholic signatory to the Declaration of Independence. Charles Jr. was the main contractor on the building of the White House. The White House and Washington city are built on lands once owned by the O'Carroll's.

The Tullamore Incident refers to a breach of law and order on 20 March 1916 in Tullamore, Co. Offaly. It is frequently referred to as 'the first shots of the Rising' although it occurred a month before the Easter Rebellion.

Dáil courts were set up in 1919 by the first Dáil. Following a court order the Tullamore Company of the IRA deported a man for stealing bales of wheat. He was sentenced to deportation, conveyed to the mailboat, and given a ticket to England.

Two members of the Offaly No. 1 Brigade of the Irish Republican Army, Patrick Geraghty and Joseph Byrne were executed by the Irish Free State during the Irish Civil War.