Warren is a Fellow of the Institute of Chartered Accountants in England and Wales (ICAEW). He has degrees in law and mathematics, and was recently awarded an MA for his research on the personal finances of Charles Dickens. He qualified as a chartered accountant with KPMG in 1990, and has been with Defries Weiss since 1995. He became a principal in 1996.

With a strong background in audit, Warren now specialises in tax planning for individuals and family businesses. He has a wide experience of representing clients in HMRC enquiries, and his main areas of expertise are in income tax and corporation tax.

Outside work, Warren is a magistrate and a keen musician.