Dr. Johnstone obtained her medical degree from Dalhousie University. She then moved to Edmonton, Alberta where she completed her Internal Medicine and Infectious Disease training at the University of Alberta. Following her residency, she completed a clinical research fellowship in Edmonton. She moved to Hamilton in July 2008 where she practised Infectious Diseases at McMaster University and completed her PhD in Health Research Methodology (Epidemiology).
Following her PhD, she joined Public Health Ontario as an Infection Prevention and Control physician where she continues to lead research focused on Infection Prevention and Control in the areas of vancomycin-resistant enterococci, ventilator-associated pneumonia and Clostridium difficile. She worked at St. Joseph's Health Center in Toronto for 5 years, where she practised Infectious Diseases and ran the Infection Prevention and Control program.
In 2018 she left St. Joseph's to become the Physician Lead of Infection Prevention and Control at Sinai Health System in Toronto where she is also an Assistant Professor in the Laboratory Medicine and Pathobiology and Dalla Lana School of Public Health, University of Toronto.