WILLIAM LONG is global co-chair of Sidley’s highly ranked Privacy and Cybersecurity practice and heads the EU Data Protection group. He is also a member of the firm’s top-ranked Crisis Management and Strategic Response team, and he serves on the Steering Committee of the firm’s AI Working Group. William advises international clients on a wide variety of AI, GDPR, cyber incident, data protection, privacy, information security, social media, e-commerce, and other regulatory matters.
William has been a member of the European Advisory Board of the International Association of Privacy Professionals (IAPP) and on the DataGuidance panel of data protection lawyers. He is also on the editorial board of e-Health Law & Policy and also assists with dplegal (“data privacy” legal), a networking group of in-house lawyers in life sciences companies examining international data protection issues.
William was previously in-house counsel to one of the world’s largest international financial services groups. He has been a member of a number of working groups in London and Europe looking at the EU regulation of e-commerce and data protection and spent a year at the UK’s Financial Law Panel (established by the Bank of England) as assistant to the chief executive working on regulatory issues with online financial services.
William has received acknowledgement from numerous industry ranking guides:
- Lexology Index 2025 – Recognized as a Thought Leader for Data.
- Chambers UK 2025 – Ranked for Data Protection and Information Law, where clients say that “he is an expert who is very pragmatic in his approach. He has a very good ability to take a risk-based approach as he has good anticipation and foresight of what regulators might think.”
- Legal 500 UK 2025 – Recognized for Data Protection, Privacy and Cybersecurity.
- Chambers UK 2024 – Ranked for Data Protection and Information Law, with sources telling the publication “he does a phenomenal job in taking a complex subject and simplifying it for us,” and “his extensive network and expertise makes him able to provide thorough yet pragmatic advice.”
- Legal 500 UK 2024 – Recognized for Data Protection, Privacy and Cybersecurity, where sources say “Client -focused. Accommodating. Well-connected within industry groups – particularly William Long.”
- BTI Client Service All-Stars 2022 – Named in this annual report for providing incomparable level of client service excellence.
William is also the author of a new book on the EU General Data Protection Regulation and a contributor to a number of books on data protection, including leading legal textbooks published by BNA in the area of privacy, cloud computing and the use of health data. William has been interviewed widely for his thought leadership, including in such leading publications as the international New York Times, Financial Times and The Guardian and writes for a number of publications, including Thomson Reuters, Law360, and Global Data Review.