Johnston later moved to the United States and taught at Mount Holyoke College, Smith College and other universities. He kept extensive diaries throughout his life, now deposited in the Library of Trinity College, Dublin, and these together with his many articles and essays give a distinctive picture of his times and the people he knew. Another archive of his work is held at the library of Ulster University at Coleraine. He received honorary degrees from the University of Ulster and Mount Holyoke College and was a member of Aosdána.