The name Strängnäs is first encountered in 1120, as the name of the Diocese. It leads back to the fact that the city is placed by a strait and on several hills, especially on two major ones, the "Mill Hill" and the "Cathedral Hill". The name "Strängnäs" is derived from Old Norsestrengr ("narrow channel of water") and nes ("isthmus", "narrow peninsula", " headland", a very common toponymic in Skandinavia). A monastery was established around 1250, and the cathedral inaugurated in 1291, and the town continued to evolve around these two institutions.
The oldest known city charter stem from 1336 by King Magnus Eriksson. Strängnäs became a city of importance in the Södermanland province, as the location of the thing and an annual market. In the 15th and 16th century, Strängnäs had an important place in the history of Sweden, the reformation, with prominent reformators Laurentius Andreae and Olaus Petri; and King Gustav Vasa elected king in Strängnäs in 1523. It attracted some education and scholarship: in 1626 the Thomas Gymnasium was established, and is today Sweden's second oldest operating gymnasium.
The development of Strängnäs seems to have been slow after that, and only flourishing temporarily with the arrival of energetic bishops. The 19th century called for industrial investments, which Strängnäs did not possess. A fire in 1871 led to large reconstructions of the city, from which time its current appearance stems.