At the age of twenty-four Thomas was given a post in the household of Theobald, archbishop of Canterbury, and while there he apparently resolved on a career in the Church, for he took minor orders. To prepare himself further, he obtained the archbishop's permission to study canon law at the University of Bologna, continuing his studies at Auxerre, France. On coming back to England, he became provost of Beverley, and canon at Lincoln and St. Paul's cathedrals. His ordination as deacon occurred in 1154. Theobald appointed him archdeacon of Canterbury, the highest ecclesiastical office in England after a bishopric or an abbacy, and began to entrust him with the most intricate affairs; several times he was sent on important missions to Rome. It was Thomas' diplomacy that dissuaded Pope Eugenius III from sanctioning the coronation of Eustace, eldest son of Stephen, and when Henry of Anjou, great grandson of William the Conqueror, asserted his claim to the English crown and became King Henry II, it was not long before he appointed this gifted churchman as chancellor, that is, chief minister. An old chronicle describes Thomas as "slim of growth, and pale of hue, with dark hair, a long nose, and a straightly featured face.
Blithe of countenance was he, winning and lovable in conversation, frank of speech in his discourses but slightly stuttering in his talk, so keen of discernment that he could always make difficult questions plain after a wise manner." Thomas discharged his duties as chancellor conscientiously and well.
Like the later chancellor of the realm, Thomas Moore, who also became a martyr and a saint, Thomas Becket was the close personal friend as well as the loyal servant of his young sovereign. They were said to have one heart and one mind between them, and it seems possible that to Becket's influence were due, in part, those reforms for which Henry is justly praised, that is, his measures to secure equitable dealing for all his subjects by a more uniform and efficient system of law. But it was not only their common interest in matters of state that bound them together. They were also boon companions and spent merry hours together. It was almost the only relaxation Thomas allowed himself, for he was an ambitious man. He had a taste for magnificence, and his household was as fine--if not finer--than the King's. When he was sent to France to negotiate a royal marriage, he took a personal retinue of two hundred men, with a train of several hundred more, knights and squires, clerics and servants, eight fine wagons, music and singers, hawks and hounds, monkeys and mastiffs. Little wonder that the French gaped in wonder and asked, "If this is the chancellor's state, what can the King's be like?" His entertainments, his gifts, and his liberality to the poor were also on a very lavish scale.
In 1159 King Henry raised an army of mercenaries in France to regain the province of Toulouse, a part of the inheritance of his wife, the famous Eleanor of Aquitaine.
Thomas served Henry in this war with a company of seven hundred knights of his own. Wearing armor like any other fighting man, he led assaults and engaged in single combat. Another churchman, meeting him, exclaimed: "What do you mean by wearing such a dress? You look more like a falconer than a cleric. Yet you are a cleric in person, and many times over in office-archdeacon of Canterbury, dean of Hastings, provost of Beverley, canon of this church and that, procurator of the archbishop, and like to be archbishop, too, the rumor goes!" Thomas received the rebuke with good humor.
Although he was proud, strong-willed, and irascible, and remained so all his life, he did not neglect to make seasonal retreats at Merton and took the discipline imposed on him there. His confessor during this time testified later to the blamelessness of his private life, under conditions of extreme temptation. If he sometimes went too far in those schemes of the King which tended to infringe on the ancient prerogatives and rights of the Church, at other times he opposed Henry with vigor.
In 1161 Archbishop Theobald died. King Henry was then in Normandy with Thomas, whom he resolved to make the next primate of England. When Henry announced his intention, Thomas, demurring, told him: "Should God permit me to be the archbishop of Canterbury, I would soon lose your Majesty's favor, and the affection with which you honor me would be changed into hatred. For there are several things you do now in prejudice of the rights of the Church which make me fear you would require of me what I could not agree to; and envious persons would not fail to make it the occasion of endless strife between us." The King paid no heed to this remonstrance, and sent bishops and noblemen to the monks of Canterbury, ordering them to labor with the same zeal to set his chancellor in the see as they would to set the crown on the young prince's head. Thomas continued to refuse the promotion until the legate of the Holy See, Cardinal Henry of Pisa, overrode his scruples. The election took place in May, 1162. Young Prince Henry, then in London, gave the necessary consent in his father's name. Thomas, now forty-four years old, rode to Canterbury and was first ordained priest by Walter, bishop of Rochester, and then on the octave of Pentecost was consecrated archbishop by the bishop of Winchester. Shortly afterwards he received the pallium sent by Pope Alexander III.
From this day worldly grandeur no longer marked Thomas' way of life. Next his skin he wore a hairshirt, and his customary dress was a plain black cassock, a linen surplice, and a sacerdotal stole about his neck. He lived ascetically, spent much time in the distribution of alms, in reading and discussing the Scriptures with Herbert of Bosham, in visiting the infirmary, and supervising the monks at their work. He took special care in selecting candidates for Holy Orders. As ecclesiastical judge, he was rigorously just.
Although as archbishop Thomas had resigned the chancellorship, against the King's wish, the relations between the two men seemed to be unchanged for a time. But a host of troubles was brewing, and the crux of all of them was the relationship between Church and state. In the past the landowners, among which the Church was one of the largest, for each hide  of land they held, had paid annually two shillings to the King's officers, who in return undertook to protect them from the rapacity of minor tax- gatherers. This was actually a flagrant form of graft and the King now ordered the money paid into his own exchequer. The archbishop protested, and there were hot words between him and the King. Thenceforth the King's demands were directed solely against the clergy, with no mention of other landholders who were equally involved.