Located in the eastern Hintertaunus and thus in the Taunus Nature Park, Ernsthausen is crossed by the Weil, a tributary of the Lahn.

[2] The oldest known documentary reference dates back to 1308, but it can be assumed that the village of Ernsthausen existed already several centuries earlier.

It is not certain whether this settlement was completely destroyed by a major fire in the Middle Ages or by devastation during the Thirty Years' War, so that no traces of it remain.

According to a document dating from 1310, the canonry of St Walpurgis in Weilburg was granted the right to collect the grain yields (taxes) from Ernsthausen.

In 1724, Count Karl August von Nassau-Weilburg acquired these rights by purchase, so that from then on the Ernsthäuser were liable to pay taxes to Weilburg.

[2] In the course of the territorial reform in Hesse, on 31 December 1970 the former market town of Weilmünster in the Oberlahn district merged voluntarily with the previously independent municipalities of Aulenhausen, Dietenhausen, Ernsthausen, Laimbach, Langenbach, Laubuseschbach, Lützendorf, Möttau, Rohnstadt and Wolfenhausen to form the new enlarged municipality of Weilmünster.