Robert became prominent because he acquired immense wealth by his marriage. He took to wife Sibilla, daughter and heiress of Robert de Ewyas, Lord Ewyas of Harold Ewyas in Hertfordshire, and in 9th of Richard I, had a suit with Herminus de Braton for the advowson of Braton church in Norfolk, part of her vast inheritance. But it appears, though duly married, a strange contest arose in the 11th of King John regarding her; when William de Newmarket was summoned to show by what right he claimed to wife her who was Robert Tregoze's wife; for it appears that Richard I had given her to Tregoze in marriage. Whereupon Newmarket came and said that he had married her in the time of Richard I by gift of Robert de Ewyas, her father. It is very certain, however, that Tregoze had sufficient power to retain the heiress, for in 14 of King John they recovered lands in Somerset, in the right of Sibella. Meanwhile Sir Robert de Tregoze was High Sheriff of Wiltshire, 3rd of Richard I, and in three years afterwards was engaged in the expedition made into Normady. In first of John 1200 he gave the King 200 marks in silver to have granted to him the wardship of the heir and lands of Geoffrey Hose, and in the 7th of John, 1206, on collecting scutage of that King's reign, answered 38 marks for 19 knight's fees, belonging to the honour of Robert de Ewyas, his father-in-law. He died about 1212. Sibilla, his wife, survived him and married 2nd Roger de Clifford and died 1236. They had two sons, daughter.