Professor Marsh is a New Zealand-trained clinical psychologist. He has held academic appointments in Clinical Psychology in Australia, Lebanon, Malaysia, New Zealand and the United Kingdom. Most of his most recent clinical work has involved with the neuropsychological assessment of individuals with traumatic brain injury, stroke, occupational exposure to neurotoxins, or suspected dementia.

His expertise within clinical psychology is in the areas of psychological assessment and research design. The majority of his research publications deal with the assessment of the psychosocial consequences of traumatic injuries or chronic illness for both the individual and their familial caregivers. He has conducted research across the life span with published studies on age groups ranging from infants to older adults. He has also published studies on non-clinical groups, primarily in the area of organisational psychology.

Professor Marsh has previously been awarded grants to conduct research on traumatic brain injury, quality of life in dialysis patients, dementia in older adults, resilience in very low-income families and internet use amongst young adults. He has served as a consultant in the areas of healthy ageing, occupational health, genetics and rehabilitation.

He is a Visiting Professor of Clinical Psychology at the Norwich Medical School of the University of East Anglia in the United Kingdom. In 2016, 2017 and 2018 he was a Visiting Professor at the University of Oviedo in Spain where he worked with the Health Sciences Research Group on School Learning, Difficulties and Academic Performance.