About 1123 Lothair of Supplinburg, then Duke of Saxony, had Blankenburg Castle erected in the Eastphalian Harzgau region.

His vassal Poppo I of Blankenburg, a relative of Lothair's wife Richenza of Northeim, is documented as count over the Eastern Harzgau since 1128.

After swearing allegiance to Dietrich, the brothers could maintain their county, their descendants laid out the Blankenburg settlement beneath the castle about 1200.

In 1599 the comital family became extinct, and the fief was reverted to the Prince-Bishopric, which at that time was held by the Welf administrator Duke Henry Julius of Brunswick and Lunenburg.

In 1690, the county was given to the younger son of Duke Anthony Ulrich of Brunswick-Wolfenbüttel, Prince Louis Rudolph.

After the apportionment of assets and liabilities between the states of the Weimar Republic and the abdicated regnal houses in 1924 the ducal family maintained - among other estates - its castle in Blankenburg am Harz and Ernest Augustus, Duke of Brunswick, and his family returned from their Austrian exile in 1930 in order to live there.

According to the London Protocol (September 1944) all of the Free State of Brunswick was supposed to become part of the future British zone of occupation in Germany.