Thomas was born in 1224 or 1225 a member of the powerful family of the Counts of Aquino at their castle of Roccasecca, in the county of Aquino, which at that time was part of the Kingdom of the Two Sicilies, half way between the lands of the Papal States and those of the Emperor Frederick the Second. His parents, Landulf and Theodora, had nine children: four boys and five girls. Thomas was the youngest of the sons and, being in that position, was appropriately destined for an ecclesiastical career. He was offered as an oblate, that is to say a monk, to the nearby Abbey of Montecassino which at that time was in a period of decadence. The offering was accompanied by a considerable sum of money. His profession, however, was conditional and not permanent. At the time Thomas was five or six years old. He remained there about ten years (1230–1239) and received an education in letters which was the basis for his religious and monastic instruction.

The Abbey's council the recommend to Landulf that he continue the education of his son Thomas at the general studium at Naples which was founded by Frederick the Second in 1224 to supply the empire with men who were able and well prepared. Between 1239 and 1244 Thomas studied first the seven liberal arts (grammar and logic) and next philosophy so that he could then advance along the path of theology. The Studium at Naples had, along with those in Palermo and Salerno, an intellectual ambience that was avant-guard and much alive with the culture of the day. The works of Aristotle on the sciences, the astronomical writings of the Arabs, and the Hippocratic science were already in circulation, thanks to translations into greek an arabic.

Beginning with his time at Naples, Thomas began a very lively adolescence. Very soon he became fascinated with the intellectual apostolate of the Dominicans who were already preaching in Naples. At the age of eighteen in 1244 he succeeded, unbeknown to is family, in being clothed with the Dominican habit. This was for him an important choice: a convent in the city replaced the isolation of a monastery; the intellectual life and preaching to people replaced a life given solely to monastic contemplation. It was a choice unacceptable to his family. His mother, Theodora, was strongly opposed to his decision. The Dominicans, accordingly, transferred Thomas to Rome, where Theodora dried in vain to catch him, and then to Bologna where he came under the jurisdiction of the Master of the Order. Theodora then asked her sons to intervene. Their expedition appeared to have had the Emperor's consent, and they succeeded in seizing Thomas and conducting him first to a castle near to Roccasecca and then to the ancestral stronghold itself. But all their efforts to get Thomas to change his mind failed. On the contrary the twenty year old Thomas profitted from his forced residence: he received visits from his religious brethren; he studied the Bible and the Sentences of Peter Lombard.

It was at this juncture, and perhaps also for political reasons, that the family backed down. After about a year, in 1245, they returned him to his brethren in Naples. From this time one Thomas began in adventurous academic career, first of all in Paris (1245-1248) where he completed the cycle of studies that had been interrupted in Naples. Within in just three years he fulfilled all the studies necessary for ordination to the priesthood and for a position of bachelor in theology. It was during this time that the frequented the lectures of the famous master Albert the Great.