Grodzisk Mazowiecki County is a unit of territorial administration and local government (powiat) in Masovian Voivodeship, east-central Poland. It came into being on 1 January 1999, as a result of the Polish local government reforms passed in 1998. Its administrative seat and largest town is Grodzisk Mazowiecki, which lies south-west of Warsaw. The county also contains the towns of Milanówek, lying north-east of Grodzisk Mazowiecki, and Podkowa Lesna, east of Grodzisk Mazowiecki.