He was also an orientalist and along with Henry Thomas Colebrooke and others he was a founding member of the Royal Asiatic Society of Great Britain and Ireland.

He became a chief justice in 1805 and in 1809 he was asked to provide suggestions for the administration of Ceylon, many of which were included in the charter for the East India Company in 1813.

[3] Johnston was responsible for bringing the Mahavamsa, Sri Lanka's historical epic, to European attention when he sent manuscripts of it and other Sinhala chronicles to Europe for publication during his tenure as Chief Justice.

[4] Jonhston encouraged the translation of the Mahavamsa and other works in order to bring British colonial law into alignment with local traditions and values.

[3] His son Alexander Robert Johnston was a colonial official in Mauritius and Hong Kong before going to England (and died in the United States in 1888).