The territory now comprising County Longford was traditionally known as Annaly (Anghaile in Irish), Tethbae or Teffia (Teabhtha in Irish) and formed the territory of the Farrell clan. After the Norman invasion of the 12th century, Annaly was granted to Hugh de Lacy as part of the Liberty of Meath. An English settlement was established at Granard, with English Cistercian monasteries being established at Abbeylara and Abbeyshrule, and Augustinian monasteries being established at Abbeyderg and at Saints' Island on the shore of Lough Ree. Monastic remains at Ardagh, Abbeylara, Abbeyderg, Abbeyshrule, Inchcleraun Island in Lough Ree and Inchmore Island in Lough Gowna are reminders of the county's long Christian history.
However, by the 14th century, English influence in Ireland was on the wane. The town of Granard was sacked by Edward Bruce's army in 1315, and the O'Farrells soon recovered complete control over their former territory.
The county was officially shired in 1586 in the reign of Elizabeth I, but English control was not fully established until the aftermath of the Nine Years War. County Longford was added to Leinster by James I in 1608 (it had previously been considered part of Connacht), with the county being divided into six baronies and its boundaries being officially defined. The county was planted by English and Scottish landowners in 1620, with much of the O'Farrell lands being confiscated and granted to new owners. The change in control was completed during the Cromwellian plantations of the 1650s.
The county was a centre of the 1798 rebellion, when the French expeditionary force led by Humbert which had landed at Killala were defeated outside the village of Ballinamuck on 8 September by an English army led by Cornwallis. Considerable reprisals were inflicted by the British on the civilian inhabitants of the county in the aftermath of the battle.
A revolutionary spirit was again awoken in the county during the Irish War of Independence when the North Longford flying column, led by Seán Mac Eoin, became one of the most active units on the Irish side during that war.