Johnson was influenced by the life of Hugh Miller, a stonemason and geologist,[1] whose books were lent to him.
Johnston obtained work on the railways, read widely, and studied botany, geology, and chemistry at the Andersonian University under Professors Kennedy, Crosskey, and Penny.
In 1880 he became chief clerk in the Audit Department, his former railway colleagues presented him with a watch inscribed: Presented to ROBERT MACKENZIE JOHNSTON by personal friends, on the occasion of his going from amongst them, in recognition not merely of his scientific attainments, but also of his social worth, and as a token of the high esteem and great regard in which he has been ever hold.
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 1903 The R. M. Johnston Memorial Volume, being a selection from his more important papers, was published by the Tasmanian government.