He was son of Henry II, Duke of Brunswick-Grubenhagen called de Graecia and his second wife Heloise (Helwig, Helvis), daughter of Philip of Ibelin, Seneschal of Jerusalem.
His father was the first son of Henry I, Duke of Brunswick-Lüneburg and his sons could succeed him in Grubenhagen, but four of them joined the church and the rest three obtained careers far away: Otto married Joanna I of Naples, Philip became Constable of Jerusalem and Balthazar entitled despot of Romania.
Additionally they had no male offspring, thus the principality of Grubenhagen remained to Ernest I, Duke of Brunswick-Grubenhagen, younger brother of Henry II.
Philip was nephew of Adelheid of Brunswick, wife of Andronikos III Palaiologos the Roman Emperor.
Because of the close relation, a Papal dispensation had been required for the marriage.