He graduated with a Bachelor of Science from the University of London and later with a Master of Science in International Policy from the School for Policy Studies, University of Bristol. He worked for Voluntary Service Overseas in Zambia, an experience which led him into a career in development where he has specialised in leadership, disability, mental health and appropriate technology.
With regard to disability and mental health, Underhill has founded a number of organisations including Thrive (formerly known as Horticultural Therapy), a UK-based charity working with disabled people and medical professionals in horticulture, gardening and agriculture, Action on Disability and Development, a development agency targeting disabled people in the third world, and BasicNeeds, which works with people with mental disorders and their carers in Ghana, Uganda, Kenya, Tanzania, India, Sri Lanka, Nepal, Lao PDR and Vietnam. Post BasicNeeds Underhill went on to co-found citiesRISE  and in 2019 he co-founded The Elders Council for Social Entrepreneurs 
Underhill has also served as chief executive of the Intermediate Technology Development Group (now Practical Action), the charity founded by E.F. Schumacher, author of Small Is Beautiful. He went on to found the UK member organization of International Development Enterprises known as IDE UK and he is currently Chair of the Ashden Awards for Sustainable Energy.
Regarding leadership notably in the Third Sector he is Member of the Board of Impetus, a leader in venture philanthropy in the UK and is the founding Chair and trustee of the Leaders' Quest Foundation, from 2004-2011. Formerly he was a board member of Headstrong, the National Centre for Youth Mental Health in Ireland, the Chair of Action Health which amalgamated with Skillshare International in 2000, Digital Links (retired July 2007), Friends of APD (founder Chair) and Oxfam Chair of Committee for Africa, Staff (HR), and Trustee Role and Council Structure working party whose task was the reorganisation of the Board of Trustees.
"Barriers to improving mental health services in low and middle income countries", and "Mental Health and Development" in Selected Readings in Community Based Rehabilitation: Disability and Rehabilitation Issues in South Asia.
In 2012 he was elected as a Senior Ashoka Fellow.
Underhill received the esteemed Skoll Award for Social Entrepreneurship 2013 which recognised his commitment towards: 'altering peace and human security practices that prevent the most marginalized or vulnerable from accessing and benefiting from the system'. The Skoll Award marked the significant contribution made by Underhill to the field of global mental health through BasicNeeds.
In 2014, he was again recognised for his contribution to the field of mental health by being selected as a Schwab Foundation Social Entrepreneur of the year. The award gives entrepreneurs the opportunity to participate in various events and initiatives of the World Economic Forum.
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- "Plants as therapy are more popular". The New York Times. 10 September 1981. Retrieved 2008-09-10.
- Coleridge, Peter (1993). Disability, Liberation, and Development. Oxfam. pp. vi. ISBN 978-0-85598-195-2. Retrieved 2008-09-10.
- "BasicNeeds: Successful Mental Health Programs With Scarce Resources". Psychiatric Times. 1 October 2006. Retrieved 2008-09-10.
- "Home". citiesRISE. Retrieved 2021-08-07.
- "The Elders Council For Social Entrepreneurs". The Elders Council For Social Entrepreneurs. Retrieved 2021-08-07.
- "Charity chief honoured for services to disabled people". Coventry Evening Telegraph. 15 June 1999. Archived from the original on 21 October 2012. Retrieved 2008-09-10.
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- "Publications". Human Issues in Horticulture. College of Tropical Agriculture and Human Resources, University of Hawai'i at Manoa. Retrieved 2008-09-10.
- Saraceno, B.; Van Ommeren, M.; Batniji, R.; Cohen, A.; Gureje, O.; Mahoney, J.; Sridhar, D.; Underhill, C. (2007). "Barriers to improving mental health services in low- and middle-income countries". The Lancet. 370 (9593): 1164–74. doi:10.1016/s0140-6736(07)61263-x. PMID 17804061. S2CID 20954187.
- Underhill, C. (2002). ""Mental Health and Development" in Selected Readings in Community Based Rehabilitation: Disability and Rehabilitation Issues in South Asia". Asia Pacific Disability Rehabilitation Journal. 2: 82–92. Archived from the original on 24 July 2011. Retrieved 1 June 2011.
- "Queen's Birthday Honours: The Full List". The Independent. London. 12 June 1999. Retrieved 2 June 2011.
- "Skoll Foundation Announces 2013 Award Winners". Skoll Foundation. Retrieved 17 January 2013.
- "The Social Entrepreneurs of the Year 2014". Schwab Foundation for Social Entrepreneurship. Retrieved 31 March 2014.