Thomas (III) was born into the Forgács branch of the wealthy and prestigious "gens" (clan) Hont-Pázmány in the 1240s. His father was Andrew (I), who erected castles near Turóc and Gímes (present-day Kláštor pod Znievom and Jelenec in Slovakia, respectively) following the First Mongol invasion of Hungary. He was a faithful confidant of Béla IV, then Stephen V. Andrew served as count of the tárnoks (financial officials) from 1249 to 1256, and "ispán" of Bánya (Árkibánya) ispánate on several occasions, which laid in the territory of Nyitra County. His mother was Maria Nánabeszter.