Philip was born in Darmstadt, as the second surviving son of George I "the Pious" of Hesse-Darmstadt (1547–1596) and his first wife, Countess Magdalene of Lippe (1552–1587).

In this case, eldest son, Louis V "the faithful" inherited the bulk of the country.

Landgrave Philip was a learned and well-traveled man; he spoke eight languages, was a mathematician and collected a valuable library.

[1] Mogling, who also translated Philip Sidney's Arcadia brought him into contact with Wilhelm Schickard (1592–1635), who had invented a calculating machine in 1623.

Philip III died in 1643 during a sweating treatment at Bad Ems, which his physician Johann Schröder had prescribed a year earlier.

Philip III suffered severe burns and died from his wounds shortly afterwards.

On 17 August 1572 Philip was married to Countess Anna Margarethe of Diepholz and Bronckhorst (1580-1629), daughter of Count Friedrich II zu Diepholz und Bronckhorst (1556-1585) and his wife, Countess Anastasia of Waldeck-Landau (1555-1582).