
Corporate M&A
Private Equity
Corporate Governance
William Allen is a Partner at Simpson Thacher & Bartlett in New York, specializing in complex public and private mergers and acquisitions and other corporate transactions. With a focus on both strategic clients and private equity sponsors, William has extensive experience in domestic and cross-border M&A, leveraged buyouts, and joint ventures across industries such as technology and financial services. He has represented high-profile clients including Microsoft, Blackstone, and KKR. William holds a JD from Harvard Law School and a BSFS from Georgetown University. Recognized as a 'Rising Star' by Law360 and The Deal, he is admitted to practice in New York.
William Allen's representative experience includes advising:

- Blackstone sold the stake/assets to Bain Capital as part of a acquisition on not available. - Deal value was 3400000000 (source: AI Estimation) Sector: Healthcare Technology; Location: USA. - On the seller side, not available advised, led by Elizabeth Cooper, William Allen, Julia Tory, Ellen Gao, Michael Lazris, Brian Gluck, Adam Moss, Callie Robins, Gregory Grogan, Caitlin Lucey, Daniella Villatoro, Caroline Geiger, Kate Mirino, Vanessa Burrows, Jacob Madden, Sophie Staples, Edward Grais, Roberto Arza, Krista McManus.
Sep 04 2025
3B
Private Equity
Partner
2021 – Unknown
Associate
2016 – 2021
Research Assistant
2011 – 2012
Summer Associate
2011 – 2011

- Blackstone Inc. sold the stake/assets to Regent as part of a M&A transaction on a date not available. - Deal value was 2200000000 (source: AI Estimation) Sector: Media, Technology; Location: Global. - On the seller side, legal counsel was not available; lawyers included Anthony Vernace, William Allen, Quinn F. Cronan, Michael L. Chen, Jack Larkin, Jonathan Goldstein, Sophie A. Staples, Shareef M. Salfity, Gregory T. Grogan, Laura R. Gallo, Josh Goldman, Alexander J. Franchilli, Kate E. Mirino, Alexander Kokka.
Mar 21 2025
2B
Private Equity
1 min
Summer Law Clerk
2010 – 2010
Intern
2008 – 2008