John Siena is Associate General Counsel and Co-Head of Global Regulatory Strategy for Brown Brothers Harriman (BBH) Investor Services, which provides asset servicing and operating model solutions to the world's leading asset managers and financial institutions. Based in London, John helps BBH and clients navigate an evolving global legal and regulatory landscape and brings deep expertise in financial services policy, market structure, and regulatory engagement. John engages with policymakers, governments, regulatory authorities, and industry participants to promote safer, more efficient capital markets that support growth, sustainability, and broader investment opportunities.

John joined BBH in 2005 as Director of Legal Operations for EMEA and rejoined the firm in 2014 following a role as Head of External and Regulatory Affairs, EMEA, at BNY Mellon. Most recently, he led BBH’s Asia legal function, providing legal and regulatory guidance across the region. Earlier in his career, he held senior legal positions at Northern Trust, supporting both investment management and custody businesses.

John is actively involved in a number of industry organizations and regulatory initiatives where he contributes to industry dialogue on regulatory developments and market infrastructure, particularly across Europe.

He sits on the Post-Trade Board and chairs the Legal Committee (Post-Trade) for the Association for Financial Markets in Europe (AFME) and chairs the European Committee of the Association of Global Custodians (AGC). He also participates in the International Securities Services Association (ISSA). John sits on UK and EU accelerated settlement industry steering committees on the ECB’s Advisory Committee for Market Infrastructure and Collateral (AMI-SeCo) and on other European and U.S. cross-sector committees with a post-trade focus.

A regular conference speaker and contributor to regulatory thought leadership, John has authored and contributed to numerous industry position papers and legislative consultations. His published work includes contributions to GDF/ISSA’s Report, Digital Asset Custody Deciphered: A Primer to Navigating the Challenges of Safeguarding Digital Assets (2023), GDF/ISDA’s Report, Unlocking Capital with U.S. Tokenized Money Market Funds for Collateral Mobility (2026), The Alternative Investment Fund Managers Directive, 3rd Edition (Kluwer Law International, 2020) and Financial Collateral (Oxford University Press, 2021). John has taught in Ireland’s Certified Investment Fund Director Institute as well as LL.M courses in financial services law at Leiden University, The Netherlands, King’s College, London, and in Chicago-Kent Law School.

John holds a BA in European History from the University of Illinois Urbana-Champaign and a JD from the University of Virginia School of Law.