The diocese is led by the Archbishop of Westminster, who serves as pastor of the mother church, Westminster Cathedral, as well as the metropolitan bishop of the ecclesiastical Province of Westminster. Since the re-establishment of the English Catholic dioceses in 1850, each Archbishop of Westminster—including the incumbent, Cardinal Vincent Gerard Nichols—has been created a cardinal by the Pope in consistory, often as the only cardinal in England, but the 43rd of English cardinals since the 12th century. It is also customary for the Archbishop of Westminster to be elected President of the Catholic Bishops' Conference of England and Wales providing a degree of a formal direction for the other English bishops and archbishops. However, he is not formally a primate, though he has special privileges conferred by the Papal Bull Si qua est. The Archbishop of Westminster has not been granted the title of Primate of England and Wales, which is sometimes applied to him, but his position has been described as that of "chief metropolitan" of the Roman Catholic Church in England and Wales and as "similar to" that of the Archbishop of Canterbury in the Church of England (as the metropolitan bishop of the Diocese of Canterbury. The diocese is one of the smallest dioceses in England and Wales in geographical area, but the largest in terms of Catholic population and priests.
The diocese essentially covers the same region as the Church of England Diocese of London as it was before the English Reformation until 1850, adopting—like all other dioceses across England (created that year)—an alternative name (originally because of the Ecclesiastical Titles Act 1851) but based on the centuries-old divisions of the country. The diocese effectively survived the period of Catholic oppression in English history as a missionary territory established by canons accepted by Rome in 1622 as the Apostolic Vicariate of England which was in public law pronounced in England and Wales illegal as counter to the established church.
The mostly clandestine apostolic vicariate covering the country was divided so that the Apostolic Vicariate of London District formed on 30 January 1688 coinciding with a degree of freedoms. By decree of Pope Pius IX (Universalis Ecclesiae), this entity gained its elevation to the rank of a metropolitan diocese (instead of archdiocese) on 29 September 1850.
On 3 April 2009, it was announced that the Archbishop of Birmingham, the Most Reverend Vincent Nichols, would become the 11th Archbishop of Westminster. Cormac Murphy-O'Connor, who was installed as tenth Archbishop of Westminster on 22 March 2000 and was elevated to the rank of cardinal-priest of the title of Santa Maria Sopra Minerva by Pope John Paul II on 21 February 2001, became archbishop emeritus. This followed Cardinal Murphy-O'Connor's announcement on 9 July 2007 that, in accordance with the age limit of 75 years prescribed for bishops in the Code of Canon Law, he had submitted his resignation to Pope Benedict XVI. He further announced that the Pope had asked him to continue in his pastoral ministry as archbishop beyond the age limit until further provision was made.