After his father's death in 1567, the Landgraviate of Hesse was divided among the four sons from the late landgrave's first marriage.

Philip the Younger received the portion around the Rheinfels Castle and city of St. Goar on the left bank of the Rhine.

The county consisted mainly of the former Lower County of Katzenelnbogen[1] with its four Ämter Rheinfels (with the city of St. Goar and the residence Rheinfels Castle) on the left bank of the Rhine, and Braubach, Reichenberg and Hohenstein on the right bank.

In 1569 Philip married Anna Elisabeth of Palatinate-Simmern, thereby becoming the son-in-law of the Elector Frederick III, one of the leaders of Calvinism.

As his marriage had remained childless, his territory fell to his elder brother Wilhelm IV, Landgrave of Hesse-Kassel (or Hesse-Cassel).