In 1645, new county lines were drawn. Medelpad and Ångermanland were combined with Jämtland to form Västernorrland County with Härnösand as its seat. Gästrikland and Hälsingland made up Härjedalden Gävleborg County, with Gävle as the county seat. Nine years later, the two counties were combined. This union lasted for more than 100 years until in 1762 the county lines were returned to their 1645 order but now with Sundsvall was the county seat of Västernorrland. At the time, the parliament´s decision had not officially assigned a county seat to Västernorrland. The county had two equally large cities, Härnösand and Sundsvall. Gustav III´s government decided in 1778 that the county administrative board would move to Härnösand. An key reason for choosing Härnösand was that the town had offered to build a county governor´s residence, unlike Sundsvall.
When Sweden gave up possession of Finland, new county lines were drawn. Västernorrland lost Jämtland, which was combined with Härjedalen to form Jämtland County.