St. Thomas was born of parents who, coming from Normandy, had settled in England some years previously. No reliance can be placed upon the legend that his mother was a Saracen.

In after life his humble birth was made the subject of spiteful comment, though his parents were not peasants, but people of some mark, and from his earliest years their son had been well taught and had associated with gentlefolk.
He learned to read at Merton Abbey and then studied in Paris.
On leaving school he employed himself in secretarial work, first with Sir Richer de l'Aigle and then with his kinsman, Osbert Huitdeniers, who was "Justiciar" of London.
Somewhere about the year 1141, under circumstances that are variously related, he entered the service of Theobald, Archbishop of Canterbury, and in that household he won his master's favor and eventually became the most trusted of all his clerks.
A description embodied in the Icelandic Saga and derived probably from Robert of Cricklade gives a vivid portrait of him at this period.
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To look upon he was 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 loveable in his conversation, frank of speech in his discourses, but slightly stuttering in his talk, so keen of discernment and understanding that he could always make difficult questions plain after a wise manner.
Theobald recognized his capacity, made use of him in many delicate negotiations, and, after allowing him to go for a year to study civil and cannon law at Bologna and Auxerre, ordained him deacon in 1154, after bestowing upon him several preferment, the most important of which was the Archdeaconry of Canterbury.
It was just at this period that King Stephen died and the young monarch Henry II became unquestioned master of the kingdom.
He took "Thomas of London", as Becket was then most commonly called, for his chancellor, and in that office Thomas at the age of thirty-six became, with the possible exception of the justiciar, the most powerful subject in Henry's wide dominions.

The chroniclers speak with wonder of the relations which existed between the chancellor and the sovereign, who was twelve years his junior. People declared that "they had but one heart and one mind".Often the king and his minister behaved like two schoolboys at play.
But although they hunted or rode at the head of an army together it was no mere comradeship in pastime which united them. Both were hard workers, and both, we may believe, had the prosperity of the kingdom deeply at heart. Whether the chancellor, who was after all the elder man, was the true originator of the administrative reforms which Henry introduced cannot now be clearly determined. In many matters they saw eye to eye.
The king's imperial views and love of splendour were quite to the taste of his minister. When Thomas went to France in 1158 to negotiate a marriage treaty, he travelled with such pomp that the people said: "If this be only the chancellor what must be the glory of the king himself?" 

In 1153 Thomas acted as justice itinerant in three counties. In 1159 he seems to have been the chief organizer of Henry's expedition to Toulouse, upon which he accompanied him, and though it seems to be untrue that the impost of "scutage" was called into existence for that Occasion (Round, "Feudal England", 268-73), still Thomas undoubtedly pressed on the exaction of this money contribution in lieu of military service and enforced it against ecclesiastics in such a way that bitter complaints were made of the disproportionately heavy burden this imposed upon the Church.
In the military operations Thomas took a leading part, and Garnier, a French chronicler, who lived to write of the virtues of St. Thomas and his martyrdom, declares that in these encounters he saw him unhorse many French nights. Deacon though he was, he lead the most daring attacks in person, and Edward Grim also gives us to understand that in laying waste the enemy's country with fire and sword the chancellor's principles did not materially differ from those of the other commanders of his time.

But although, as men then reported, "he put off the archdeacon", in this and other ways, he was very far from assuming the licentious manners of those around him. No word was ever breathed against his personal purity. Foul conduct or foul speech, lying or unchastely were hateful to him, and on occasion he punished them severely.
He seems at all times to have had clear principles with regard to the claims of the Church, and even during this period of his chancellor ship he more than once risked Henry's grievous displeasure. For example, he opposed the dispensation which Henry for political reasons extorted from the pope, and strove to prevent the marriage of Mary, Abbess of Romsey, to Matthew of Boulogne.
But to the very limits of what his conscience permitted, Thomas identified himself with his master's interests, and Tennyson is true to history when he makes the archbishop say: 

I served our Theobald well when I was with him: 

I served King Henry well as Chancellor: 

I am his no more, and I must serve the Church.

Archbishop Theobald died in 1161, and in the course of the next year Henry seems to have decided that it would be good policy to prepare the way for further schemes of reform by securing the advancement of his chancellor to the primacy.Our authorities are agreed that from the first Thomas drew back in alarm. "I know your plans for the Church," he said, "you will assert claims which I, if I were archbishop, must needs oppose."
But Henry would not be gainsaid, and Thomas at the instance of Cardinal Henry of Pisa, who urged it upon him as a service to religion, yielded in spite of his misgivings.He was ordained priest on Saturday in Whit week and consecrated bishop the next day, Sunday, 3 June, 1162.
It seems to have been St. Thomas who obtained for England the privilege of keeping the feast of the Blessed Trinity on that Sunday, the anniversary of his consecration, and more than a century afterwards this custom was adopted by the papal Court, itself and eventually imposed on the whole world. 

A great change took place in the saint's way of life after his consecration as archbishop. Even as chancellor he had practiced secret austerities, but now in view of the struggle he clearly saw before him he gave himself to fasting and disciplines, hair shirts, protracted vigils, and constant prayers.
Before the end of the year 1162 he stripped himself of all signs of the lavish display which he had previously affected. On 10 Aug. he went barefoot to receive the envoy who brought him the pallium from Rome. Contrary to the king's wish he resigned the chancellorship. Whereupon Henry seems to have required him to surrender certain ecclesiastical preferment's which he still retained, notably the archdeaconry, and when this was not done at once showed bitter displeasure.
Other misunderstandings soon followed. The archbishop, having, as he believed, the king's express permission, set about to reclaim alienated estates belonging to his see, a procedure which again gave offence. Still more serious was the open resistance which he made to the king's proposal that a voluntary offering to the sheriffs should be paid into the royal treasury.

As the first recorded instance of any determined opposition to the king's arbitrary will in a matter of taxation, the incident is of much constitutional importance. The saint's protest seems to have been successful, but the relations with the king only grew more strained. Soon after this the great matter of dispute was reached in the resistance made by Thomas to the king's officials when they attempted to assert jurisdiction over criminous clerks.
That the saint himself had no wish to be lenient with criminous clerks has been well shown by Norgate (Angevin Kings, ii, 22). It was with him simply a question of principle. St. Thomas seems all along to have suspected Henry of a design to strike at the independence of what the king regarded as a too powerful Church.
With this view Henry summoned the bishops at Westminster (1 October, 1163) to sanction certain as yet unspecified articles which he called his grandfather's customs (avitæ consuetudines), one of the known objects of which was to bring clerics guilty of crimes under the jurisdiction of the secular courts. The other bishops, as the demand was still in the vague, showed a willingness to submit, though with the condition "saving our order", upon which St. Thomas inflexibly insisted.
The king's resentment was thereupon manifested by requiring the archbishop to surrender certain castles he had hitherto retained, and by other acts of unfriendliness. In deference to what he believed to be the pope's wish, the archbishop in December consented to make some concessions by giving a personal and private undertaking to the king to obey his customs "loyally and in good faith".

But when Henry shortly afterwards at Clarendon (13 January, 1164) sought to draw the saint on to a formal and public acceptance of the "Constitutions of Clarendon", under which name the sixteen articles, the avitæ consuetudines as finally drafted, have been commonly known, St. Thomas, though at first yielding somewhat to the solicitations of the other bishops, in the end took up an attitude of uncompromising resistance.
Then followed a period of unworthy and vindictive persecution. When opposing a claim made against him by John the Marshal, Thomas upon a frivolous pretext was found guilty of contempt of court. For this he was sentenced to pay £500; other demands for large sums of money followed, and finally, though a complete release of all claims against him as chancellor had been given on his becoming archbishop, he was required to render an account of nearly all the moneys which had passed through his hands in his discharge of the office. Eventually a sum of nearly £30,000 was demanded of him.His fellow bishops summoned by Henry to a council at Northampton, implored him to throw himself unreservedly upon the king's mercy, but St. Thomas, instead of yielding, solemnly warned them and threatened them.

Then, after celebrating Mass, he took his archiepiscopal cross into his own hand and presented himself thus in the royal council chamber. The king demanded that sentence should be passed upon him, but in the confusion and discussion which ensued the saint with uplifted cross made his way through the mob of angry courtiers. He fled away secretly that night (13 October, 1164), sailed in disguise from Sandwich (2 November), and after being cordially welcomed by Louis VII of France, he threw himself at the feet of Pope Alexander III, then at Sens, on 23 Nov.
The pope, who had given a cold reception to certain Episcopal envoys sent by Henry, welcomed the saint very kindly, and refused to accept his resignation of his see. On 30 November, Thomas went to take up his residence at the Cistercian Abbey of Pontigny in Burgundy, though he was compelled to leave this refuge a year later, as Henry, after confiscating the archbishop's property and banishing all the Becket kinsfolk, threatened to wreak his vengeance on the whole Cistercian Order if they continued to harbor him.The negotiations between Henry, the pope, and the archbishop dragged on for the next four years without the position being sensibly changed. Although the saint remained firm in his resistance to the principle of the Constitutions of Clarendon, he was willing to make any concessions that could be reasonably asked of him, and on 6 January, 1169, when the kings of England and France were in conference at Montmirail, he threw himself at Henry's feet, but as he still refused to accept the obnoxious customs Henry repulsed him. At last in 1170 some sort of reconciliation was patched up. The question of the customs was not mentioned and Henry professed himself willing to be guided by the archbishop's council as to amends due to the See of Canterbury for the recent violation of its rights in the crowning of Henry's son by the Archbishop of York. On 1 December, 1170, St. Thomas had brought with him, as well as over the restoration by the de Broc family of the archbishop's castle at Saltwood.

The story that Henry VIII in 1538 summoned the archbishop to stand his trial for high treason, and that when, in June, 1538, the trial had been held and the accused pronounced contumacious, the body was ordered to be disinterred and burnt, is probably apocryphal.