After ordination as a deacon in 1952, Westwood was appointed curate of Holy Trinity Church, Kingston upon Hull. He was then Rector of Lowestoft; Vicar of St Peter Mancroft, Norwich (1965–75) and an honorary canon of Norwich Cathedral. He became the Bishop suffragan of Edmonton in the Diocese of London from his consecration on 24 June 1975 by Donald Coggan, Archbishop of Canterbury, at St Paul's Cathedral. From the creation of the London area scheme in 1979, he was the first area bishop, remained in that See until his translation to Peterborough in late 1984. He was enthroned at Peterborough Cathedral on 12 January 1985, but had become Bishop of Peterborough before that point, with the confirmation of his canonical election to that See near the end of 1984. He retired as Bishop of Peterborough in December 1995.
He was the chairman of the Church of England's Committee for Communications, President of the Church Housing Association, a member of the Board of Governors of Nene College and an honorary fellow of Emmanuel College, Cambridge from 1989 until his death.
Westwood was a regular contributor to Thought for the Day on BBC Radio 4's Today programme. He was a member of the Press Council (1975–81), the IBA Panel of Religious Advisers (1983–7), the Video Consultative Council (1985–9) and the Broadcasting Standards Council (1988–92).
- "No. 54456". The London Gazette. 4 July 1996. p. 9013.
- ‘WESTWOOD, Rt Rev. William John’, Who Was Who, A & C Black, an imprint of Bloomsbury Publishing plc, 1920–2015; online edn, Oxford University Press, 2014
- "No. 46592". The London Gazette. 6 June 1975. p. 7289.
- "New suffragans consecrated (Archived; subscription only)". Church Times (#5863). 27 June 1975. p. 3. ISSN 0009-658X. Retrieved 9 March 2016. (Subscription required (help)).
- "4: The Dioceses Commission, 1978–2002" (PDF). Church of England. Archived from the original (PDF) on 7 June 2012. Retrieved 23 April 2013.
- "New bishop tells diocese 'be open' (Archived; subscription only)". Church Times (#6362). 18 January 1985. p. 3. ISSN 0009-658X. Retrieved 9 March 2016. (Subscription required (help)).
- "Advert: a video for Christmas! (Archived; subscription only)". Church Times (#6357). 14 December 1984. p. 19. ISSN 0009-658X. Retrieved 9 March 2016. (Subscription required (help)).
- "Signs of a resilient Church (Archived; subscription only)". Church Times (#6930). 8 December 1995. p. 4. ISSN 0009-658X. Retrieved 9 March 2016. (Subscription required (help)).
|Church of England titles|
| Bishop of Edmonton (London)
| Bishop of Peterborough
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