The present municipality was created in 1971, when the City of Strängnäs was amalgamated with the City of Mariefred and a number of rural municipalities. Originally there were 15 local government entities in the area.
Strängnäs is an old town with a history dating back to the Viking era. Its location on the shores of Lake Mälaren has made Strängnäs an important trading centre and meeting place through the ages.
Mariefred acquired its name from the monastery Pax Mariae ("Mary's Peace") which was founded some 500 years ago by Sten Sture the Elder. There are no remains above ground of the monastery, which was the location where Mariefred's church, built in 1624, now stands.