Thomas was a kaid, or magister, of the royal Diwan throughout the reign of Roger II, whose favour he certainly had. His name appears in many official documents and it was probably he who drafted the foundation charter of the Cappella Palatina. He is known to have had a secretary named Othman, named in an Arabic source. It is interesting that he appears in charters in Latin, Greek, and Arabic. The Greek transcription of his name and title was μαστρο Θωμα του Βρουνου.