Philip was the third son of Landgrave Philip the Magnanimous and Christine of Saxony (1505–1549). After his father's death in 1567, the Landgraviate of Hesse was divided between the four sons out of the late landgrave's first marriage. Philip the Younger received the portion around the castle Rheinfels and city of St. Goar on the left bank of the Rhine. From there he controlled the towns of Schotten, Stornfels, Bad Homburg, Lißberg, Ulrichstein, Itter, and the part of the former high noble and remarkably rich County of Katzenelnbogen north of the river Main.
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.
Philip died on 30 November 1583 on castle Rheinfels. As his marriage had remained childless, his territory fell to his elder brother Wilhelm IV, Landgrave of Hesse-Kassel (or Hesse-Cassel). Philip was buried in St. Goar, where Wilhelm erected an imposing Renaissance monument.