He was the son and heir of Humphrey II de Bohun (died 1164/1165) of Trowbridge Castle and of Caldicot Castle, 4th feudal baron of Trowbridge,[1] by his wife Margaret of Hereford, a daughter of Miles FitzWalter of Gloucester, 1st Earl of Hereford, Lord of Brecknock (died 1143), Sheriff of Gloucester and Constable of England, by his wife Sibyl de Neufmarché.
By 29 September 1165 he had succeeded to his father's estates, when he owed three hundred marks as feudal relief for the barony.
Later that same year he and Richard de Lucy led the sack of Berwick-upon-Tweed and invaded Lothian to attack William the Lion, King of Scotland, who had sided with the rebels.
He returned to England and played a major role in the defeat and capture of Robert Blanchemains, Earl of Leicester, at Fornham.
[2] It has been suggested that Humphrey's wife was the Margaret who married Pedro Manrique de Lara, a Spanish nobleman, but there are discrepancies in the theory.