LANCASHIRE was established in 1182 and later than many other counties. During Roman times the area was considered part of the Brigantes tribal area and was in the military zone of Roman Britain. The towns of Manchester, Lancaster, Ribchester, Burrow, Elslack and Castleshaw were all originally Roman forts. In the centuries immediately following the Roman withdrawal in 410AD the northern parts, at least, of the county probably formed part of the Brythonic kingdom of Rheged, a successor entity of the Brigantes tribe of northern Britain. During the mid 8th Century this area was incorportated into the Anglo-Saxon Kingdom of Northumbria which itself became a part of England in the 10th Century. In the Domesday Book, its lands between the Ribble and the Mersey were known as "Inter Ripam et Mersam" and were included in the returns for Cheshire. Although some historians have taken this to mean that south Lancashire was, at that time, part of Cheshire, neither can it be said clearly to have been part of Cheshire.  It is also claimed that the territory to the north formed, at that time, part of the West Riding of Yorkshire. It bordered on Cumberland, Westmorland, Yorkshire, and Cheshire. The county was divided into the six hundreds of Amounderness, Blackburn, Leyland, Lonsdale, Salford and West Derby. Lonsdale was further partitioned into Lonsdale North, which was the detached part north of "the sands" of Morecambe Bay (including both Furness and Cartmel), and Lonsdale South.

Lancashire is now much smaller than its historical extent, following a major reform of local government. In 1889 an administrative county of Lancashire was created, covering the historical county except for county boroughs such as Blackburn, Burnley, Barrow-in-Furness, Preston, Wigan, Liverpool, Manchester. The area covered by the Lord-Lieutenant (termed now a ceremonial county) continued to cover the entirety of the administrative county along with the county boroughs, and thus was expanded slightly whenever boroughs annexed areas in other neighbouring counties. Examples of this include Wythenshawe (an area of Manchester south of the River Mersey and historically in Cheshire), and southern Warrington. This area also did not cover the western part of Todmorden, where the ancient border between Lancashire and Yorkshire runs through the middle of the town.

During the 20th century the county became increasingly urbanised, particularly the southern part. To the existing county boroughs of Barrow-in-Furness, Blackburn, Bolton, Bootle, Burnley, Bury, Liverpool, Manchester, Oldham, Preston, Rochdale, Salford, St Helens and Wigan were added Blackpool (1904), Southport (1905), and Warrington (1900). The county boroughs also had many boundary extensions. The borders around the Manchester area were particularly complicated, with narrow protrusions of the administrative county between the county boroughs – Lees urban district formed a detached part of the administrative county, between Oldham county borough and the West Riding of Yorkshire.