Storme Sixeas joins Grant Thornton as Tax Legislative Affairs leader

Grant Thornton, one of America’s largest brands of professionals providing end-to-end audit, assurance, tax and advisory services, today announced that Storme Sixeas has joined the firm’s Washington National Tax Office (WNTO) as its new Tax Legislative Affairs leader.

In her role, Sixeas will navigate the intersection of tax policy and government, serving as a trusted advisor to clients by providing guidance on tax legislative matters. With more than two decades of experience in Washington, Sixeas is adept at delivering critical insights on federal tax policy and the political landscape to a broad range of industries and taxpayers.

Mark Margulies, Grant Thornton’s national managing principal of Tax Services, shared his enthusiasm about Sixeas joining the team and the expertise she brings to the role.

“In today’s ever-evolving tax legislative landscape, bringing an in-depth point of view and adaptable solutions is essential to staying ahead of trends and ultimately delivering exceptional client service,” said Margulies. “Storme’s experience as a trusted tax adviser, paired with her legislative and advocacy background, uniquely positions her to help our clients navigate complexity and drive meaningful results.”

Prior to joining Grant Thornton, Sixeas spent nearly a decade in the Washington National Tax office at Deloitte, where she served as an expert adviser to large clients, tax practitioners and other stakeholders on federal tax policy and the political landscape. Before that, she excelled as a policy lobbyist for UK defense firm BAE Systems, Inc., and the Electronic Industries Alliance, a high-tech trade association; and was a leadership aide in the House of Representatives.

“I’m excited to join Grant Thornton and contribute to a team that’s at the forefront of tax policy and client service,” said Sixeas. “I look forward to helping companies of all sizes map the lay of the land where tax policy and politics meet with as much clarity as possible in today’s dynamic legislative environment.”

David Sites, national managing principal and head of Grant Thornton’s WNTO and International Tax Solutions, echoed Margulies’ sentiment.

“Storme brings a rare combination of policy acumen and practical legislative experience to our team,” said Sites. “Her ability to translate developments on Capitol Hill into clear, actionable guidance will be a tremendous asset to our clients and colleagues alike.”

Sixeas is the past chair of the Tax Coalition, a professional organization that brings together women in tax policy across Capitol Hill, the administration and the private sector. She is also the most recent past U.S. chair of the British-American Project, a transatlantic leadership network. Sixeas received an MBA from George Mason University and a bachelor’s degree in journalism from the University of Missouri.

To learn more about Grant Thornton’s tax service offerings, visit: https://www.grantthornton.com/services/tax-services.

About Grant Thornton

Grant Thornton delivers professional services in the US through two specialized entities: Grant Thornton LLP, a licensed, certified public accounting (CPA) firm that provides audit and assurance services ― and Grant Thornton Advisors LLC (not a licensed CPA firm), which exclusively provides non-attest offerings, including tax and advisory services.

In January 2025, Grant Thornton formed a multinational, multidisciplinary platform with Grant Thornton Ireland. The platform offers a premier advisory and tax practice, as well as independent American and Irish audit practices. In April 2025, Grant Thornton UAE also joined the multinational platform. With $2.7 billion in revenues and almost 60 offices spanning the U.S., Ireland, the UAE and other territories, the platform delivers a singular client experience that includes enhanced solutions and capabilities, backed by powerful technologies and a roster of almost 13,000 quality-driven professionals enjoying exceptional career-growth opportunities and a distinctive cross-border culture.

Grant Thornton is part of the Grant Thornton International Limited network, which provides access to its member firms in more than 150 global markets.

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