Johnston visited the Ethiopian Empire (then known as Abyssinia) in 1842 – 1843 and recorded his experience in a book titled Travels in Southern Abyssinia, Through the Country of Adal to the Kingdom of Shoa. His book forms an important historical account of the customs and culture of the region in the 1840s. He befriended the King of the Ethiopian kingdom of Shewa, Sahle Selassie, publishing in his book their conversations from various meetings. After his travels in Ethiopia he returned to England where he became assistant editor of the Lady's Newspaper. Some years later Johnston returned to Africa to settle in Durban, Colony of Natal (now part of South Africa), where he founded the Durban Botanic Gardens, which is now the oldest surviving botanic gardens in Africa. Johnston returned to England in 1861 and eventually settled in Barnstaple, Devon.
