In 1882 Johnston was appointed registrar-general and government statistician. Johnston was appointed a royal commissioner to report on the fisheries of Tasmania, being the author of "Descriptive Catalogue of Tasmanian Fishes" (Hobart, 1882).
In 1882, Johnston took on the responsibilities of registrar-general and government statistician. He was also appointed to investigate Tasmania's fisheries as a royal commissioner, leading to his publication "Descriptive Catalogue of Tasmanian Fishes" (Hobart, 1882).