He received his episcopal consecration on 24 January 1992 from Cardinal Basil Hume, OSB, with Archbishop Derek Worlock and Bishop Alan Clark serving as co-consecrators, at Westminster Cathedral.

[4] He selected as his episcopal motto: Fortis Ut Mors Dilectio, meaning, "Love Is Strong As Death" Song of Solomon 8:6.

On 15 February 2000, Nichols was appointed the eighth Archbishop of Birmingham by Pope John Paul II, succeeding the French-born Maurice Couve de Murville.

Prior to his appointment to Birmingham, he had been considered a leading contender to replace the late Cardinal Hume as Archbishop of Westminster;[13] the position went to Murphy-O'Connor, although Nichols would later succeed him.

He is assisted in this role by two further episcopal trustees – Archbishop Arthur Roche, Secretary of the Congregation for Divine Worship and the Discipline of the Sacraments, and Mark Jabalé, Bishop Emeritus of Menevia.

By virtue of his status as ordinary of the Birmingham diocese, Nichols played a leading role in the cause of the Canonisation of John Henry Newman, which took place in 2019.

Nichols has written two books: Promise of Future Glory and Missioners; and it was he who set up the "Walk with Me" programme, which sought to bring people together in spiritual accompaniment through the seasons of the Church’s year.

[17] A group of English Catholic bishops, as well as a member of parliament, had even expressed their concerns of promoting Nichols to Westminster to the Apostolic Nuncio, Faustino Sainz Muñoz, citing the archbishop's ambition.

[17][25] Elena Curti of The Tablet suggested this was the result of advice early in his career from Archbishop Worlock to make himself more "Vatican-friendly" if he was to get ahead in the Church hierarchy.

The Big Society [...] has no teeth [...] and should not be used as a cloak for masking central cuts"[29] Nichols defended the ban of the church on Communion for the divorced and remarried.

Among his first public acts in his role as Archbishop of Westminster was a statement on the issue of clerical physical and sexual abuse in Ireland following a government report into the running of industrial schools.

He was, however, criticised widely on the issue of priests facing up to their crimes, where he claimed, "That takes courage, and also we shouldn't forget that this account today will also overshadow all of the good that they also did.

"[31] In 2020 the Independent Inquiry into Child Sexual Abuse said of Nichols, now a cardinal and the senior Catholic cleric in England and Wales, "There was no acknowledgement of any personal responsibility".

[4] In 2004, he prominently intervened in an argument pitching religious offence against artistic freedom when he criticised Birmingham Repertory Theatre for showing the play Behzti (Dishonour), which depicted scenes of sexual abuse and murder in a Sikh temple.

[37] On 20 October 2009, Cardinal William Levada and Archbishop Joseph Di Noia held a press conference in which they announced that Pope Benedict XVI was preparing to release an apostolic constitution that would allow Anglicans, both laity and clergy, to join the Catholic Church in groups and maintain their corporate identity in new Personal Ordinariates for former Anglicans entering the Roman Catholic Church.

[38] A joint statement on the new protocol from Nichols and the Anglican Communion's head, Archbishop Rowan Williams of Canterbury, was issued at the same time in London.

For from the Anglican perspective, this new invitation to swim the Tiber can sometimes have a slightly predatory feel; in corporate terms, a little like a takeover bid in some broader power play of church politics.

"[40] Following the ascent of Arthur Roche to the Congregation for Divine Worship and the Discipline of the Sacraments and his "clarifying" Responsa ad Dubia supplement to Traditionis custodes, Nichols moved to suppress and prohibit the celebration of the Sacrament of Confirmation in the Archdiocese of Westminster according to the traditional rites of the Catholic Church, as they were celebrated before the institution of the Novus Ordo in 1971 (created after Vatican II).

[41] This affected the annual traditional Catholic confirmations at St James's, Spanish Place in London (part of the archdiocese of Westminster), where, before 2022, they had been celebrated under indult for the last 20 years.

In February 2024 Nichols announced that the traditional mass planned for the Paschal Triduum, that had annually been held at St. Mary Moorfields, would be canceled.

Mary Ann Sieghart, a journalist, commenting for The Times on Nichols' statements on the subject, observed that "had the Catholic position been more hardline, it might have stood more of a chance.

[46] Nichols also told teachers at Catholic schools that they could not marry divorced people, enter into civil unions or same-sex marriages and yet retain their jobs.

[47] In 2013, Cardinal Nichols, under pressure from the Vatican, put an end to Masses for LGBT people at the historic 18th-century Our Lady of the Assumption Church in Warwick Street, Soho.

Cardinal Nichols arranged, however, for the LGBT community to move to the Church of the Immaculate Conception, Farm Street in nearby Mayfair and attended their first Mass there in 2013.

Archbishop Nichols said: "The moral teaching of the Church is that the proper use of our sexual faculty is within a marriage, between a man and a woman, open to the procreation and nurturing of new human life.

"[49] In 2006, Nichols denounced then-Secretary for Education Alan Johnson's plan to introduce a quota for non-Catholic pupils at Catholic schools as "insulting", "divisive" and "ill-thought-out, unworkable and contradictory of empirical evidence".

(…) there are growing numbers of hungry pupils in classrooms across the [UK] country and many accounts of parents foregoing their own meals to provide for their children.

[55] Nichols argued in 2014 that government changes to social security had "torn apart" the "basic safety net" for very poor families and called them a "disgrace".

[57] During a visit to the port of Tilbury in June 2015, Cardinal Nichols paid tribute to seafarers' professionalism and dedication and the sacrifices they make to support their families.