She teaches auditing, and her research investigates auditor decision making, including auditors' client acceptance and continuance decisions, how fraud risk and fraud brainstorming affects audit planning and audit fees, client-auditor negotiation, and audit budget-setting processes. She has also published various articles on accounting curriculum effectiveness. Professor Johnstone serves on the editorial boards of several academic journals and is active in the Auditing Section of the American Accounting Association, currently serving on the Executive Committee in the role of Treasurer. She has worked in practice as a corporate accountant, a staff auditor, and was a doctoral fellow in residence at Coopers and Lybrand.