Although Johnston seems only to have published one article in India (on a group of medieval statues), his later works show that he had noted local Indian practices in agriculture and other areas, since he made reference to these in his analysis of Sanskrit texts. Between 1928 and 1936, he published an edition and translation of the Buddhacārita (Acts of the Buddha) by the 2nd-century author Aśvaghoṣa; this was described by the writer of his obituary in The Times as his "magnum opus."
In 1937, he was elected Boden Professor of Sanskrit and Keeper of the Indian Institute at the University of Oxford, also becoming a Professorial Fellow of Balliol College. He started cataloguing the Sanskrit manuscripts acquired for the Bodleian Library by an earlier Boden professor, A. A. Macdonell, helped improve the museum of the Indian Institute, and worked on the manuscripts held by the India Office Library. He published several articles on a variety of topics. (Source Accessed Jan 13, 2020)