Stewart has supported the armed forces charity Combat Stress since learning about his father's post-traumatic stress disorder when researching his family genealogy for the documentary series Who Do You Think You Are? He is patron of the United Nations Association – UK, and delivered a speech at UNA-UK's UN Forum 2012 on Saturday 14 July 2012, speaking of his father's experiences in the Second World War, and how he believed the UN was the best legacy of that period.
