The first record of the term "Cumberland" appears in 945, when the Anglo-Saxon Chronicle recorded that the area was ceded to Malcolm I by King Edmund of England. At the time of the Domesday Book in 1086 most of the future county remained part of Scotland although some villages in the ancient district of Millom, which were the possessions of the Earl of Northumbria, were included in the Yorkshire section with the Furness region.
In 1092 King William Rufus of England invaded the Carlisle district, settling it with colonists. He created an Earldom of Carlisle, and granted the territory to Ranulf Meschyn. In 1133 Carlisle was made the see of a new diocese, largely identical with the area of the earldom. However, on the death of King Henry I in 1135, the area was regained by Scotland's King David I. He was able to consolidate his power and made Carlisle one of his chief seats of government, while England descended into a lengthy civil war. In 1157 Henry II of England resumed possession of the area from Malcolm IV of Scots, and formed two new counties from the former earldom: Westmorland and "Carliol" although Westmorland also included areas the former Honour or Earldom of Lancaster. The lead and silver-mining area of Alston, previously associated with the Liberty of Tynedale was later also added to the new county of Carliol for financial reasons. By 1177 the county of Carliol was known as Cumberland. The border between England and Scotland was made permanent by the Treaty of York in 1237.
The boundaries formed in the 12th century did not change substantially over the county's existence. It bordered four English counties and two Scottish counties. These were Northumberland and County Durham to the east; Westmorland to the south, the Furness part of Lancashire to the southwest; Dumfriesshire to the north and Roxburghshire to the northeast.
To the west the county was bounded by the Solway Firth and the Irish Sea. The northern boundary was formed by the Solway Estuary and the border with Scotland running east to Scotch Knowe at Kershope Burn. The boundary ran south from Scotch Knowe along the Cheviot Hills, then followed a tributary of the River Irthing and crossed Denton Fell to the River Tees. From Tees Head the boundary crossed the Pennines to descend Crowdundale Beck, from where it followed the rivers Eden and Eamont to the centre of Ullswater. The line then followed the Glencoin Beck to the top of the Helvellyn ridge, thence to Wrynose Pass and along the River Duddon to the sea near Millom.