On 3 May 2012, Warner's appointment as the next diocesan Bishop of Chichester was announced,[9] his election was confirmed on 2 July[10] and his enthronement took place in Chichester Cathedral on 25 November.[11]
In a compromise by John Sentamu, Archbishop of York, to whom the candidate professed canonical obedience, Warner officiated as principal celebrant in the laying-on of hands and Eucharist for the episcopal consecration of Philip North as suffragan Bishop of Burnley at York Minster on 2 February 2015 (Feast of Candlemas).[12]
On 20 November 2012, Warner was one of three bishops (and one of the two diocesans) in the General Synod who voted against a motion to allow the ordination of women as bishops in the Church of England.[14] His position on the issue has been criticised by some local clergy and parishioners across the Chichester diocese.[15]
He is a member of the Council of Bishops of The Society, an association of traditionalist Anglo-Catholics.[16]
In 2023, following the news that the House of Bishop's of the Church of England was to introduce proposals for blessing same-sex relationships, he signed an open letter which stated:[17]
many Christians in the Church of England and the Anglican Communion, together with Christians from across the churches of world Christianity, continue to believe that marriage is given by God for the union of a man and woman and that it cannot be extended to those who are of the same sex. [...] Without seeking to diminish the value of many committed same-sex relationships, for which there is much to give thanks, we find ourselves constrained by what we sincerely believe the Scriptures teach which cannot be set aside.[17]
He voted against introducing "standalone services for same-sex couples" on a trial basis during a meeting of the General Synod in November 2023; the motion passed.[18][19]
^"GENERAL SYNOD NOVEMBER 2023 GROUP OF SESSIONS BUSINESS DONE AT 5 P.M."(PDF). churchofengland.org. The Church of England. 13 November 2023. Retrieved 23 November 2023. 7. The motion (as amended) 'That this Synod, conscious that the Church is not of one mind on the issues raised by Living in Love and Faith, that we are in a period of uncertainty, and that many in the Church on all sides are being deeply hurt at this time, recognise the progress made by the House of Bishops towards implementing the motion on Living in Love and Faith passed by this Synod in February 2023, as reported in GS 2328, encourage the House to continue its work of implementation, and ask the House to consider whether some standalone services for same-sex couples could be made available for use, possibly on a trial basis, on the timescale envisaged by the motion passed by the Synod in February 2023.' was carried following a counted vote by Houses.