The present municipality was formed in 1971 through the amalgamation of the City of Flen (instituted in 1949), the market town (köping) Malmköping and the surrounding countryside, including a southeastern portion of the former Oppunda hundred that was split between Flen and Nyköping municipalities.

[3] The highest peak is at 103 metres (338 ft) on the tripoint between Flen, Gnesta and Strängnäs municipalities.

This is a demographic table based on Flen Municipality's electoral districts in the 2022 Swedish general election sourced from SVT's election platform, in turn taken from SCB official statistics.

[19] As of 2022 it was a rather segregated municipality with very high levels of ethnic minority populations concentrated south of the railway, while people of Swedish background are in a sizeable majority in the outer parts of town and in the countryside.

Every district was beneath the median income of its southern neighbour Nyköping.