Diane P. Sullivan

Chambers USA notes: “Diane Sullivan is one of the best trial lawyers of all time,” ranking her for over twenty years in its “Star” category as one of the few truly elite trial lawyers in the country. Similarly, The American Lawyer  featured Diane as a “Litigator of the Year,” noting she “isn’t a hired gun; she’s more like a hired bazooka repeatedly parachuting into high-stakes cases leading up to trial and securing victories.” Benchmark Litigation describes her as “one of the nation’s most iconic trial lawyers.” Legal 500 remarked: “Noted for her superb advocacy skills and ability to explain often complex technical/scientific concepts to juries, she is frequently parachuted in to act as trial counsel on the eve of important trials.” She has been featured for her trial victories on the front pages of The New York Times, The Wall Street Journal, and on the cover of The American Lawyer, among other publications.

Diane’s high-stakes jury verdicts have also been featured in four different years by The National Law Journal in its annual series: “Winning: Successful Strategies from the Nation’s Top Litigators,” which highlights significant trial wins by the nation’s leading trial lawyers. The same publication highlighted her jury verdicts as among the “Top Verdicts of the Year” in seven different years. Clients described her to Chambers as “a trial powerhouse” and “a renowned courtroom presence, equally adept at handling the full range of complex commercial cases, mass torts and class actions.” Clients note: “she is a “superstar presence in the courtroom”; “a fantastic and fearless trial lawyer at the top of her game.” Clients further note she is an “absolutely sterling, extraordinary trial lawyer,” “fantastic,” “a top choice in bet-the-company litigation,” and “one of the best I’ve ever seen.” “She controls a courtroom and is scared of nothing and no one.” “She is a visionary, very bold and proactive, focusing on delivering for her clients.” “A consummate trial lawyer – there is nobody more focused on the jury than Diane.” “She has a mastery of the courtroom. Clients further advise: “She is a first class trial lawyer in every way,” “a natural on her feet,” “a real weapon at trial,” and “one of the best trial lawyers in the country.” “She is one of the best trial lawyers out there; we go to her with our most important work.” With experience across a range of industries, she is praised as “a truly tremendous trial lawyer” and “a superstar litigator” who “keeps on winning ‘nationally significant cases’ in jurisdictions throughout the country.” Clients further remarked: “Her trial presence is unmatchable,” and “she was just wonderful – a powerful presence who boils down complex issues in a way that can be readily communicated to a jury.” “Her ability to connect with jurors in difficult jurisdictions stands out.” Chambers also noted: “She’s rightly named a top lawyer – she handles some of the toughest cases.” Clients praised her “remarkable talent,” “amazing cross-examinations,” and “fantastic ability to build up a narrative and connect with juries in an honest way.” They also note that she “gets great results” and is “fearless, excellent, and as zealous an advocate as there is in the Bar.” In 2021, Courtroom View Network listed her among its “Trial Powerhouse Pros,” recognizing the ten most effective civil jury trial lawyers in the country.

Diane is also named as a “Lawdragon Legend” for being recognized as one of the “500 Leading Lawyers in America” for ten years or more (now 19 consecutive years). Lawdragon notes: “she has produced a batting average in jury trials in the Hall of Fame category” and describes her as “one of the most feared trial lawyers in the nation.” The publication further stated: “In courts or arbitrations for corporate clients, she is as good as any advocate in the country.” Lawdragon also recognizes her as an “All Star Litigator” which recognizes the top litigators in the country, and in 2025 they selected her to their 2025 “Leading Litigators in America” list.

Diane was also named one of Law360’s “Trial Aces” recognizing the top trial attorneys in the country based on success in high-stakes trials, and as one of Law360’s Trial “MVPs” in the publication’s ranking of the five most valuable trial lawyers in the country based on significant trial victories. Diane was also selected as one of Law360’s Litigation “MVPs” for success in major litigation and has been repeatedly recognized as a Law360 “Legal Lion” for success in high profile litigations. Diane was also highlighted by Law360 as one of the most admired attorneys in the nation based on a survey of her peers and in-house counsel. A member of The Legal 500 “Hall of Fame” for lawyers at “the pinnacle of the profession,” the publication notes that she is “an absolute star,” “a terrific trial lawyer,” “a gifted trial lawyer in ‘big ticket, bet-the-company trials’ who regularly secures favorable verdicts in high-stakes matters.” She is rated by clients as “just a brilliant, brilliant trial lawyer;” “renowned for her ability to try complex commercial, antitrust, patent, mass tort, and class action cases in jurisdictions all across the US.” She is “in a small class of bet-the-company trial lawyers.” She received the prestigious Benchmark Litigation “Hall of Fame” award and has been repeatedly listed on Benchmark’s Top 100 Trial Lawyers list. She is also recognized by Legal Media Group as one of the “Best of the Best” USA recognizing the top 30 practitioners in the field, and in 2023 and 2024 was named a Leading Commercial Litigator by Daily Journal.

Lawyers Weekly USA named her one of the top 10 lawyers of the year, noting her trial performance “electrified the courtroom.” Diane was included in the Financial Times’ top 100 female champions in its “HERoes: Champions of Women in Business” ranking. The International Who’s Who references Diane for commendation by her peers as one of the most highly regarded practitioners in the field. Diane received the Euromoney Women in Business “Most Outstanding Practitioner” award, given to one lawyer each year for her contributions to the legal profession. Diane was also named one of the top 15 Female Trial Attorneys by Law360 for scoring “landmark victories” and “blazing trails.” She has been repeatedly recognized by Benchmark Litigation as a “Litigation Star” and as one of the Top Ten Women Litigators in the country. LMG Life Sciences Guide named Diane a “Life Science Star” and praised her as a “nationally revered trial lawyer who has routinely secured victories, often high stakes and precedent-setting, for a ‘Who’s Who’ list of clients.” LMG also awarded her “Trial Lawyer of the Year,” “Impact Case of the Year,” and “Product Liability Attorney of the Year” honors at its annual Life Sciences Awards.

Among Diane’s notable trial victories and representations are:

• Lead trial counsel for Altria in a patent infringement case against R.J. Reynolds involving the best-selling e-vapor product; successfully securing a unanimous jury verdict for $95 million in pre-judgment damages and potentially over $1 billion in future royalties.
• Lead trial counsel for defendant in the Astroworld litigation pending in Texas state court MDL involving thousands of claims including for wrongful death and personal injury arising out of a Travis Scott concert.
• Lead trial counsel for Frontier Communications in a False Claims Act case in West Virginia where the government sought hundreds of millions in damages relating to a fiber optic cable contract that spanned the state; achieved favorable settlement on eve of jury selection.
• Lead trial counsel for pharmacy chain defendant in the first bellwether jury trial against pharmacies in the massive MDL opioid litigation where two plaintiff Ohio counties sought over three billion dollars in damages, achieving a successful settlement of all pending cases against our client at close of plaintiffs’ case.
• Lead trial counsel for Johnson & Johnson securing a unanimous defense verdict following a four-week bellwether jury trial in New Jersey state court involving claims J&J’s baby powder caused cancer. This was the first defense verdict in New Jersey courts, where thousands of cases in the talc mass tort were pending against the Company. The verdict, described as a “slam dunk” for the defense, helped to “turn the tide” in the litigation, according to Reuters and The American Lawyer, which featured the verdict in its “Litigator of the Week” series. Law360 also highlighted the win as a landmark victory of the year. Courtroom View Network showcased the win as the most impressive defense verdicts of the year.
• Lead trial counsel for GE securing a complete defense win after a four-week arbitration in a suit involving claims for over $1 billion in damages relating to construction of a clean energy power plant.
• Lead trial counsel for ESPN securing a defense verdict against Dish Network following a three-week jury trial in the Southern District of New York involving more than $150 million in damage claims. This victory was featured in The American Lawyer and in Law360’s “How They Won It” series.
• Lead trial counsel for Merck in the legal battle over the prescription pain medicine Vioxx® involving more than 50,000 claims nationwide. Diane secured the first trial victory for Merck, convincing jurors to reject the plaintiff’s failure to warn and consumer fraud claims. This defense verdict, in a plaintiff-friendly “judicial hell-hole” jurisdiction, was featured on the front pages of newspapers across the country and InsideCounsel named it one of the “five verdicts of 2005 that are reshaping corporate defense strategies.” The National Law Journal featured the win as one of the top verdicts of the year. Subsequently, Diane scored another victory for Merck in the Vioxx® litigation – the first defense verdict in a long-term use case. The victory was hailed by analysts as significant, particularly in serving to thwart future collateral estoppel arguments. This verdict was also featured by The National Law Journal in its annual “Winning” series featuring the top verdicts of the year. She went on to win yet a third jury verdict for Merck in a Vioxx® case; a victory also featured in legal and lay publications.
• Lead trial counsel for Philip Morris USA securing a unanimous jury defense verdict following a three-week class action jury trial of medical monitoring claims pursued on behalf of all Massachusetts smokers of Marlboro cigarettes who had smoked for 20-plus pack-years, with a total class size in the tens of thousands. This verdict was featured in The American Lawyer “Litigator of the Week” series, in Law360, and in The National Law Journal “Winning” series as a top verdict of the year.
• Lead trial counsel for global energy company Repsol in environmental litigation involving claims of “alter ego” and fraudulent conveyance. The Weil team secured a victory on all claims involving over $1 billion dollars in damages while also succeeding on Repsol’s $65 million counterclaim. The win was profiled in Law360.
• Lead trial counsel for Philip Morris USA securing a defense verdict after a four-month jury trial in St. Louis where 37 Missouri hospitals sued for over $500 million dollars in economic loss plus punitive damages relating to costs for treating patients with smoking-related illness. The victory was described by the press as “resounding and complete.” Diane’s work in this trial was highlighted by the Financial Times as an example of “innovative lawyering” and was also recognized by The American Lawyer in its “Litigator of the Week” profile and in The National Law Journal’s Winning series as a top verdict of the year.
• Lead trial counsel for Tarkett in class action jury trial involving artificial turf fields for sports stadiums achieving a favorable settlement on the eve of trial.
• Lead trial counsel for Procter & Gamble in a consumer fraud class action litigation relating to its probiotic supplement Align^® securing a favorable settlement on the eve of trial.
• Lead trial counsel for Procter & Gamble in litigation brought by several New York water authorities alleging 1,4 dioxane groundwater contamination relating to the company’s signature Tide laundry detergent.
• Lead trial counsel for Sanofi in antitrust litigation against Mylan relating to the marketing of EpiPen^®
• Lead trial counsel for SAC Capital and Steven Cohen in litigation brought by Fairfax Financial Holdings for $8 billion dollars alleging fraud and conspiracy relating to short sales of securities; summary judgment granted for Mr. Cohen and for SAC Capital before trial. The win was highlighted in newspapers across the country and featured in Law360.
• Lead trial counsel for Walgreens in consumer class action litigation relating to Theranos Labs blood testing technology, securing dismissal of core claims against Walgreens and an indemnification from Theranos.
• Lead trial counsel for a national office supply company in a high profile antitrust case brought by the Federal Trade Commission to block a multi-billion dollar merger.
• Lead counsel for Sanofi securing a dismissal of all claims in Illinois federal court in a consumer class action following Sanofi’s global recall of its epinephrine injection product, Auvi-Q^®. The win was featured in Law360.
• Lead counsel for a transportation equipment leasing company in an international mass tort in Lac-Mégantic, Quebec, Canada arising from a major train derailment involving environmental, property and wrongful death claims in multiple forums securing a favorable settlement and a channeling injunction preventing future claims.
• Lead trial counsel for AstraZeneca securing a defense verdict in the first bellwether trial of more than 20,000 claims involving the prescription drug Seroquel®. The verdict was described by the press as a “huge win” for AstraZeneca, particularly the jury’s finding that AstraZeneca’s warnings were adequate. The trial victory led to a global settlement of all claims.
• Lead trial counsel for Cardinal Health Corporation in patent infringement litigation relating to gel capsule technology. Secured favorable settlement on eve of trial.
• Lead trial counsel for Merck and Schering-Plough in class action lawsuits alleging consumer fraud and RICO claims relating to the drugs Vytorin^® and Zetia^®; favorably settled before trial.
• Lead trial counsel for a major Japanese manufacturer in federal False Claims Act suits involving hundreds of millions of dollars in potential damages; secured favorable settlement on eve of trial.
• Lead trial counsel for Baxter Healthcare Corporation and Allegiance Healthcare Corporation in the first trial in the federal MDL latex medical device mass tort litigation, which resulted in a defense jury verdict. The verdict was named one of the top defense verdicts of the year by The National Law Journal.
• Lead trial counsel for Nutraquest in the ephedra mass tort litigation, including defending a $685 million personal injury claim filed by the estate of professional baseball player Steve Bechler. Secured favorable global settlement on eve of trial.

Diane also has an active pro bono practice. She negotiated a groundbreaking settlement with the New York City Housing Authority (NYCHA) in a class action lawsuit she commenced on behalf of domestic violence survivors, challenging NYCHA’s administration of housing assistance in the wake of domestic abuse. The settlement, reported in Politico and other publications, remedies a system that deprived survivors’ due process rights and placed them in danger of eviction. As a result of Diane’s advocacy, domestic violence survivors now have the means of obtaining new housing vouchers in their names, safeguarding them against the loss of their homes.

Diane is a frequent author and speaker on topics relating to litigation and trial practice. She has served on the Firm’s Management Committee for successive terms.