[2] The University of St Andrews awarded her a first class honours degree in Medieval and Modern History in 1951 and she additionally studied Philosophy and Latin.

[2] She stayed on to complete a PhD at St Andrews in 1956, with a thesis entitled ‘The Government of Ireland 1767–85: A study in Anglo-Irish Political Administration’; this was later developed into the book, Great Britain and Ireland 1760–1800: a study in political administration.

[2] After completing her PhD, Johnston gained a Diploma in Education from Queens University Belfast.

[2] Johnston researched Irish history and that became her expertise and she specialised in its parliament and administration in the eighteenth-century.

In 1976, Johnston became a Professor and Chair of Modern History at Macquarie University in Sydney, Australia.