Whilst studying at King's College, he undertook many sketching trips in England and on the Continent - an important part of every architect's education at that time.
[2] Articled to John Belcher, Johnston was notable for his sensitive conservation and restoration work on many churches in the south-east of England and other historic buildings.
[2] In Surrey his professional work included the churches at Cater- ham, Chaldon, Charlwood, Compton, Coulsdon, Ewehurst, Stoke D'Abemon, Warlingham, and Witley.
His brother was Sir Henry Hamilton (Harry) Johnston, a British explorer, botanist, artist, colonial administrator, linguist.
[6] PM Johnston married at Yapton parish church, Sussex, on 3 September 1902, Florence Anna Wynne, third daughter of the Right Rev.
Publications A Schedule of Antiquities in the County of Surrey, 1913, Surrey Archeological Society Send Church and the Chapel of Ripley, 1901, Roworth & Co. Shorthampton Chapel and its wall-paintings, 1905, Harrison An Early Window and Wall Paintings in Witley Church,1918, Roworth & Co. Church Chests of the twelfth and thirteenth centuries in England, 1908, Harrison & Sons Old Camberwell: its history and antiquities, 1919, J.R.Wigzell White Swan Hotel, Stratford on-Avon, etc., 1928, Trust Houses Church of S. Mary, Stoke D'Abernon, 1930, Spottiswoode, Ballantyne & Co. Photographs by Johnstone are held at the Conway Library of art and architecture in the Courtauld, London.