Formerly Professor of Welsh at Swansea University, he has published extensively on Welsh literature of all periods, including some of the English-language writers of Wales. He edited the volume on the 20th century in the University of Wales Press’s Guide to Welsh Literature series, and wrote the volume on the literature of Wales in its ‘Pocket Guide’ series.
His main research interest is medieval Welsh poetry, and he has specialized in textual editing, including three major editions, Gwaith Iolo Goch (1988), Gwaith Lewys Glyn Cothi (1995) and Gwaith Llywelyn Goch ap Meurig Hen (1998), as well as two ground-breaking thematic collections, Medieval Welsh Erotic Poetry(1991) and Poets’ Grief (1993). In 2005 he published a comprehensive study of late-medieval Welsh literature, Llên yr Uchelwyr: Hanes Beirniadol Llenyddiaeth Gymraeg 1300–1525 , a volume which was short-listed for the Welsh Book of the Year Prize.
Professor Johnston was director of an interdepartmental research project funded by the AHRC (2001–6) which produced a new edition of the poetry of Dafydd ap Gwilym in electronic format, http://www.dafyddapgwilym.net/ .
At present he is co-investigator on the Guto’r Glyn Project at the Centre. He has been co-editor of the journal Studia Celtica since 2002, and he was a member of the Celtic Studies Sub-panel for RAE 2008.