Its administrative seat and largest town is Sandomierz, which lies 83 kilometres (52 mi) east of the regional capital Kielce.

The county covers an area of 675.89 square kilometres (261.0 sq mi).

The recorded history of the Sandomierz County area dates back to the mid-14th century, when King Kazimierz Wielki (reigned 1333–1370) reformed the territorial structure of Poland.

Since Sandomierz had already been one of the most important cities of the kingdom, and the seat of a castellan, the castellany was replaced by a county.

Stretched along both banks of the Vistula, in 1667 it included such towns, as Opatow, Polaniec, Staszow, Wachock, Rudnik nad Sanem, Zawichost, Nowa Slupia, Mielec and Ozarow.