 MacLean spent a large amount of time of his working life at Yale Medical School and the National Institutes of Health where through his research he was able to publish Neuroscience texts such as reports, photographs and audio-visual material on his neurological findings.MacLean spent two years during World War 2 serving as a medical officer for the Yale Unit, which later became known as the 39th General Hospital, this experience helped to shape MacLean's perspective on the impacts of Post-Traumatic Stress Disorder on fallen soldiers, which would ultimately shape his future studies into the way the human brain functions and how it can be easily damaged through life experiences, with particular focus on sleeping disorders and other mental health issues, including anxiety and depression.MacLean had a deep fascination with the natural human instinct, and the role that the brain plays with rational human thinking.