A former law clerk to Justice Antonin Scalia of the U.S. Supreme Court and to Judge Diarmuid F. O’Scannlain of the Ninth Circuit, Daniel Sullivan is a true generalist, whose practice ranges across trial litigation and appeals over numerous subject matters, postures, and aspects of the profession. Described by clients as “bright, incisive, deep, and relentless,” Dan has led or played a key role in cases in forums around the country, as well as leading arbitrations.
A leader of HSG’s appellate practice, Dan has led or played a key role in cases in fora around the country, including state and/or federal courts in Arkansas, California, Delaware, Florida, Georgia, Illinois, Missouri, Nevada, New Jersey, New York, North Carolina, Ohio, Rhode Island, and West Virginia, as well as leading arbitrations before JAMS and AAA. He is currently leading or has argued appeals in the Second, Seventh, and Eleventh Circuits, as well as New York’s First and Third Departments and Court of Appeals, the Supreme Courts of Delaware, Nevada, and West Virginia, and the Arkansas Court of Appeals.
Among other significant matters, Dan recently prevailed in a landmark ruling in Delaware Supreme Court, solidifying a key defense for insurers against coverage claims for opioid lawsuits. Dan also led the appeal on behalf of opt-out investors following the first ever trial of direct shareholder claims in Nevada state court. Last year, he argued an appeal in the West Virginia Supreme Court raising novel questions regarding when an anti-suit injunction is appropriate. He also won an order from the U.S. Supreme Court vacating a Second Circuit decision in an appeal with significant implications for insider trading prosecutions. He led two appeals for TIG Insurance Company in the Second Circuit addressing issues of arbitrability of disputes and the standard for vacating conflicted decisions, which resulted in a partial victory for TIG on the way to a favorable settlement while TIG’s petition for certiorari was pending. This month he will argue an appeal in New York’s First Department testing the limits of the continuing wrong and equitable estoppel doctrines, as applied to the statute of limitations, in a managing-agent dispute under New York law.
Dan also has had a leading role in trial-level commercial litigation, including representing Chubb in approximately 20 insurance coverage actions nationwide arising out of the national opioid epidemic, managing RMBS putback cases covering billions of dollars in loans, representing a significant renewable company in a dispute over a failed solar power purchase agreement, securities fraud matters, partnership breakdowns, earnout disputes, and other business litigation. Dan was also appointed to serve as plaintiffs’ liaison counsel in a high-profile multi-district litigation concerning issues arising from pregnant women’s prenatal use of acetaminophen, such as Tylenol.
Dan is a member of the advisory board of the Institute of Judicial Administration at NYU School of Law, a prestigious group of judges and appellate advocates recognized for their contributions to the justice system and has been a regular panelist for the New York Federal Bar Council’s annual Supreme Court preview program since 2018. He was named a Rising Star and a Trailblazer by the New York Law Journal, an Elite Boutique Trailblazer by The National Law Journal, and a Litigation Star by Benchmark Litigation.
Updated Sep 2025
