The City of Fagersta (instituted in 1944) was in 1967 merged with Västervåla parish (which had been part of Surahammar Municipality since 1963). In 1971 it was transformed into a municipality of unitary type.
Fagersta is known in Sweden for being a stronghold of Vänsterpartiet (Left party). In the last municipal elections, in 2006, vänsterpartiet got 58,30% of the votes. This outcome was the most successful, nationally, for vänsterpartiet in the municipal elections. Further this makes Fagersta the only Swedish municipality with the former communist party in a majority. The success for the party locally is ascribed to the popular local frontsman Stig Henriksson.