Philip was the son of Guy of Milly, a knight from Picardy who participated in the First Crusade, and his (possibly second) wife Stephanie of Flanders. Guy and Stephanie had three sons, all born in the Holy Land, of whom Philip was probably the oldest. He was first mentioned as Guy's son in 1138, and must have become lord of Nablus sometime between that date and 1144, when his name next appears, or maybe in 1142. By this time he had also married his wife Isabella.
As lord of Nablus, Philip became one of the most influential barons in the Kingdom of Jerusalem. In 1144, Queen Melisende sent him to relieve the siege of Edessa, but he arrived after the city had already fallen. In 1148, upon the arrival of the Second Crusade, Philip participated in the council held at Acre, where he and the other native barons were overruled and the ill-fated decision to attack Damascus was made.
Along with the powerful Ibelin family, into which his half-sister Helvis had married, Philip was a supporter of Melisende during her conflict with her son Baldwin III. In the division of the kingdom in 1151, Melisende gained control of the southern part of the kingdom, including Nablus. Despite this arrangement, Philip seems to have been completely loyal to Baldwin, participating in the king's capture of Ascalon in 1153 and the relief of Banyas in 1157.
In July of 1161, as Melisende lay dying, Philip exchanged the lordship of Nablus with Baldwin III in order to become lord of Oultrejordain. This allowed Baldwin to regain control of the southern half of the kingdom while his mother was in no condition to oppose him, but he was probably also aiming to strengthen Oultrejordain with a powerful and loyal baron. Baldwin died in 1163 and was succeeded by his brother Amalric, who was a friend of Philip and a fellow supporter of Melisende during the earlier struggle in 1151.
Philip's personal life is largely a mystery, but it is known that sometime after he became lord of Oultrejordain, he made a pilgrimage the monastery of St. Catherine's on Mount Sinai. With his wife Isabella he had a son, Rainier (who predeceased him), and two daughters, Helena and Stephanie. Isabella died probably in 1166, which apparently caused Philip to take vows as a brother of the Knights Templar. His lands were inherited by his elder daughter, Helena, wife of Walter III of Brisebarre, lord of Beirut, and (after her death in 1168) by her daughter, Beatrice of Brisebarre.
Philip joined Amalric's invasion of Egypt in 1167. The Ibelin family later recalled an event during the siege of Bilbeis, in which Philip saved the life of Hugh of Ibelin, who had broken his leg when his horse fell in a ditch, although the veracity of this story is unknown. The Templars as a whole refused to support Amalric's invasion, and the king blamed them for the failure of the expedition. After the death of their Grand Master Bertrand de Blanchefort in January of 1169, Amalric pressured them to elect Philip in his place, which they did in August of that year. Not much is known about Philip's time as Grand Master, although he likely led the defense of Templar-held Gaza when Saladin, who had gained control of Egypt in 1169, attacked the city in 1170.
For unknown reasons he resigned as Grand Master in 1171, and was succeeded by Odo de St Amand. Philip accompanied Amalric to Constantinople as ambassador to the Byzantine Empire in order to restore good relations with them after the failure of the Egyptian invasion. He probably died on April 3, before reaching Constantinople.