Williamson was born in Buckinghamshire, England, the middle son of a Marks and Spencers buyer and his wealthy American wife. Williamson attended Winchester College and then went to Clare College, Cambridge where he received a degree in English literature. Upon graduating, he taught at a college in Ghana for a brief period. In September 1965 he returned to England and from 1965 to 1970 taught at St Paul's School in London, where records show him to have been a popular teacher and speaker, involved with multiple extracurricular activities.

Williamson, originally an Anglican, was received into the Catholic Church in 1971. After a few months as a novice at the Brompton Oratory, Williamson left. Williamson is fluent in French, German and Spanish. He became a member of the Society of St Pius X a traditionalist Catholic faction founded by Archbishop Marcel Lefebvre in 1970 in protest of the liberalism of the second Vatican council, He entered the International Seminary of St. Pius X at Ecône Switzerland, and in 1976 he was ordained a priest by Lefebvre.

Williamson subsequently moved to the United States where he served as the rector of St. Thomas Aquinas Seminary in Ridgefield, Connecticut from 1983, and continued in the position when the seminary moved to Winona, Minnesota in 1988.

After his episcopal consecration Williamson remained rector of St. Thomas Aquinas Seminary. He performed various episcopal functions, including confirmations and ordinations. In 1991, he assisted in the consecration of Licínio Rangel as bishop for the Priestly Society of St. John Mary Vianney after the death of its founder, Bishop Antônio de Castro Mayer. In 2006, he ordained two priests and seven deacons in Warsaw, Poland for the Ukrainian Greek Catholic Priestly Society of Saint Josaphat.

In 2003 Williamson was appointed rector of the Seminary of Our Lady Co-Redemptrix in La Reja, Argentina.

Catholicism

In common with his colleagues in the Society of St. Pius X, Williamson opposes the changes in the Catholic Church brought about by the Second Vatican Council. He sees such changes as being unacceptably liberal and modernistic, and as being destructive to the Church. Among the changes he opposes are the Church's increased openness to other Christian denominations and other religions, and changes in the forms of Catholic worship such as the general replacement of the Tridentine Mass with the Mass of Paul VI. Williamson has criticised Pope John Paul II, to whom he attributed a "weak grasp of Catholicism". Williamson holds that the SSPX is not schismatic, but rather is composed of true Catholics who are keeping the "complete Roman Catholic apostolic faith".

Williamson is viewed as being located towards the hardline end of the traditionalist spectrum, though he does not go so far as to espouse sedevacantism. In the past, he opposed compromise between the SSPX and the Church leadership in Rome, accusing the latter of deceit and of being under "the power of Satan". He has been reported as viewing reconciliation between the SSPX and the Holy See as being impossible, and he has noted that some SSPX members might refuse to follow the Society in such a direction even if an agreement were reached.

Society

Williamson holds strong views regarding gender roles and dress. He opposes women wearing of trousers or shorts, attending college or university, or having a career, and has urged greater "manliness" in men.

He supports authoritarian parenting style, denouncing the film The Sound of Music as "soul-rotting slush" and saying that, by putting "friendliness and fun in the place of authority and rules, it invites disorder between parents and children."

Williamson supports conspirary theories regarding the assassination of President Kennedy, and the World Trade Center controlled demolition conspiracy theory, denying that the latter were terrorist attacks but were instead staged by the U.S. government.

Jews and holocaust denial

Williamson has expressed controversial views about Jews. He called them the "enemies of Christ" and urges their conversion to Catholicism. He claims that Jews and Freemasons have contributed to the "changes and corruption" in the Catholic Church. He has also stated that Jews aim at world dominion and believes The Protocols of the Elders of Zion to be authentic. Williamson has denied that he is promoting hatred, asserting that "I was attacking the enemies of our Lord Jesus Christ, and that includes Jews as well as Communists and Freemasons." He argues that "Anti-Semitism means many things today, for instance, when one criticizes the Israeli actions in the Gaza Strip. The Church has always understood the definition of anti-Semitism to be the rejection of Jews because of their Jewish roots. This is condemned by the Church."

Since the late 1980s, Williamson has been accused of antisemitism and Holocaust denial. Citing the pseudoscientific Leuchter report, Williamson has denied that millions of Jews were murdered in Nazi concentration camps and the existence of Nazi gas chambers and praised Holocaust denier Ernst Zündel. During an interview on Swedish television recorded in November 2008, he stated: "I believe that the historical evidence is strongly against, is hugely against six million Jews having been deliberately gassed in gas chambers as a deliberate policy of Adolf Hitler", and "I think that 200,000 to 300,000 Jews perished in Nazi concentration camps, but none of them in gas chambers."

Wishing to heal the rift with the SSPX, Pope Benedict XVI lifted the excommunications of the four bishops Lefebvre had consecrated. The decree was signed on January 21, 2009, the same day that the interview on Swedish television was broadcast. The decision stirred widespread outrage, particularly in Germany, where the interview was conducted and where Holocaust denial is illegal and punishable by imprisonment of up to five years. Reaction from the State of Israel and much of the worldwide Jewish community was strongly negative, and Abraham Foxman, president of the Anti-Defamation League, wrote to Cardinal Walter Kasper in order to express his opposition to any ecclesiastic re-integration of Bishop Williamson. In January 2009, the Chief Rabbinate of Israel suspended contacts with the Vatican. The Chief Rabbi of Haifa told the Jerusalem Post that he expected Williamson to publicly retract his statements before any dialogue could resume. On 4 February 2009, German prosecutors announced the launch of a criminal investigation into the statements. In October 2009, a German court, using an "order of punishment" fined Williamson ?12,000 after finding him guilty of holocaust denial. Williamson denied the charges and appealed, paving the way for a full hearing that Williamson need not attend. He did not attend the trial, on orders from his society, on charges of inciting racial hatred in Regensburg, Germany on April 16, 2010, and was found guilty. The court reduced the fine to .

Pope Benedict XVI stated that he deplored all forms of antisemitism and that all Catholics must do the same. The Pope expressed his "unquestionable solidarity" with the Jewish people, and stated his hope that "the memory of the Shoah will induce humanity to reflect on the unpredictable power of hate when it conquers the heart of man", and condemned the denial of the Holocaust. Vatican officials stated that they had not been aware of Williamson's views prior to the lifting of the excommunication; as a result, in a July 2009 Vatican reorganization, the Pope tightened control and supervision over reconciliation efforts with SSPX.

Williamson sent the Pope a letter expressing his regret about the problems that he had caused, but did not retract his statements. On 4 February 2009 the Vatican Secretariat of State issued a note stating that Williamson would have to distance himself unequivocally and publicly from the opinions that he had expressed before he would be permitted to act as a bishop within the Church. Williamson responded that he would do so only after looking at the historical evidence for himself.On 26 February, he formally apologized for the offense that had been caused by his comments, but did not indicate that he had changed his views. The Vatican rejected his apology, stating that he needed to "unequivocally and publicly" withdraw his comments. Some Jewish groups have expressed disappointment at the ambiguity of his apology, because he failed to address the factuality of the Holocaust. He subsequently repeated the denial to followers, stating that "The fact is that the 6 million people who were supposedly gassed represent a huge lie."

Bishop Fellay of the SSPX initially denied responsibility, stating that Williamson's statements were his alone and that the affair did not concern the SSPX as a whole. However, he subsequently forbade Williamson from speaking out publicly about historical or political matters, and asked Pope Benedict for forgiveness for the damage done by Williamson's statements. In a subsequent interview he likened Williamson to uranium, asserting that "It's dangerous when you have it," but you can't "simply leave it by the side of the road.

Williamson was removed as the head of the seminary of in La Reja, Argentina and Bishop Fellay stated said that if Williamson again denied the Holocaust, he would be excluded from the Society.

In February 2009, the government of Argentina asked Williamson to leave the country over irregularities with his visa, and stating that his recent statements about Jews “profoundly offend the Argentinean society, the Jewish people and all of humanity". On 24 February 2009 Williamson flew from Argentina to London, where he was met by Michele Renouf, a former model known for her antisemitic views, with whom he had been put in touch by holocaust denier David Irving.