After several unification efforts starting in 1902, the two boroughs of Hartlepool and West Hartlepool merged into a single county borough called Hartlepool in 1967, also absorbing at the same time the neighbouring parish of Seaton (being the residual rural part of the old parish of Seaton Carew) to provide coastal land for industrial development.

The borough was reformed and enlarged on 1 April 1974, by the merger of the previous county borough of Hartlepool, along with the parishes of Brierton,