St Johnston worked as an official in Fiji, and lived in Queensland at one stage. He wrote several boys' adventure books on the South Pacific, including Camping Among Cannibals (1883), Charlie Asgarde: The Story of a Friendship (1884), Twycross' Redemption: A Story of Wild Adventure (1887), and the descriptive romance of native life and customs, A South Sea Lover: A Romance (1890).