Professor McLennan began his professional career as an industrial/organisational psychologist, first with the Navy and then with the Commonwealth public service. He subsequently moved, via occupational psychology, into counselling psychology, in which field he worked for some 25 years as practitioner, educator, and researcher.
In the course of his research, much of which involved telephone-based help services, he became increasingly interested in how people made decisions: both to seek, and to offer, help. This rekindled his interest from early days in safety-related decision making and in the mid-1990s, in collaboration with Dr Mary Omodei, he began to work with the Melbourne Metropolitan Fire Brigade investigating decision-making by on-scene Incident Controllers. This led to various research studies concerning decision making with the Defence Science and Technology Organisation - Air Operations, and Marine Operations, the Office of Corrections, and CFA.
In 2004 he joined the School as a senior Research Fellow, managing the Bushfire Cooperative Research Centre (CRC) Volunteerism Project and working on the Firefighter Safe Behaviour and Decision Making Project. Since the 2009 Black Saturday Victorian bushfires he has been conducting research in community bushfire safety.