It is 44 kilometers (27 mi) long,[1] starts below the settlement of Velika Preska,[2] flows through the Mirna Valley and joins the Sava at Dolenji Boštanj, opposite Sevnica.

The river was mentioned for the first time in 1028 in relation to a 1016 document by Henry II, Holy Roman Emperor.

[3] Mirna is the river in which the marathon swimmer Martin Strel first learned to swim.

This is attested by medieval transcriptions of the name containing the letter n (e.g., inter fluenta Nirine in 1016).

The expert on Slovene language Ivan Gregorčič and the geographer Maja Topole have characterised the latter form as incorrect.