Johnston was born in Auckland, New Zealand. He was educated at Seddon Memorial Technical College and St John's College, Auckland  before beginning his ordained ministry with a curacy at St Mark’s Remuera. He then had incumbencies at Dargaville, Northern Wairoa and Otahuhu. In 1949 he became Archdeacon of Waimate, a position he held for four years before being appointed the Bishop of Dunedin. He was consecrated a bishop on 24 February 1953. He was translated to be Bishop of Waikato in 1969 and was additionally elected Archbishop of New Zealand in 1972. He served as a member of the Royal Commission to Inquire into and Report upon the Circumstances of the Convictions of Arthur Allan Thomas for the Murders of David Harvey Crewe and Jeanette Lenore Crewe.
- London Gazette
- "Obituary: Allen Howard Johnston". The New Zealand Herald. 23 February 2002. Retrieved 16 September 2011.
- “Who was Who” 1897-2007 London, A & C Black, 2007 ISBN 978-0-19-954087-7
- Crockford's Clerical Directory1940-41 Oxford, OUP,1941
- "Archived copy" (PDF). Archived from the original (PDF) on 9 June 2011. Retrieved 24 August 2013.CS1 maint: archived copy as title (link)
|Anglican Communion titles|
William Alfred Robertson Fitchett
| Bishop of Dunedin
Walter Wade Robinson
John Tristram Holland
| Bishop of Waikato
Norman Alfred Lesser
| Archbishop of New Zealand
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