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.

When did the county become more urbanised?