The Irish name for the county reflects its pre-Norman history; it was part of the territory of Uí Failghe, a tribal grouping whose name may be continued in the modern surname Faley or Fally. In historical times the most powerful families in the region were the O'Carrolls (who gave their name to Ely O'Carroll, an area in the south of the county), the O'Connors and the O'Molloys. Their lands were annexed to the English crown in the thirteenth century, but effective English control was not imposed until the sixteenth century, when county was planted with English settlers and renamed King's County, to match its neighbour Queen's County, now Laois. The counties acquired their present names after independence in 1922.
With the exception of the Slieve Bloom mountains which form its southern boundary, the terrain of the county is mostly flat, with many large peat bogs; these now supply turf for the county's power stations.
Charles Carroll, a signatory of the American Declaration of Independence, was descended from an Offaly family, as is Oliver North, well known for his associations with Nicaragua and Iran..
Surnames associated with the county include Dooley, Dunne, Egan, Dempsey, Lalor, Flattery, Daly, Condron and Lynam.