It came into being on 1 January 1999, as a result of the Polish local government reforms passed in 1998.

Its administrative seat and only town is Nidzica, which lies 48 kilometres (30 mi) south of the regional capital Olsztyn.

The county covers an area of 960.7 square kilometres (370.9 sq mi).

The county's population has stagnated and remained the same for the past four decades, never surpassing the 35,300 mark.

In 1825, Nidzica County (Kreis Neidenburg) had 29,617 inhabitants, including by mother tongue: 27,467 (~93%) Polish and 2,149 (~7%) German.