While Stanford's Dean, Sullivan maintained a pro bono constitutional law practice. In 2005, she joined Quinn Emanuel Urquhart Oliver & Hedges (now Quinn Emanuel Urquhart & Sullivan), where she helped to found and build the firm's nascent appellate practice. Now chair of the firm's national appellate practice, Sullivan has argued and briefed numerous appeals before the U.S. Courts of Appeals, the California Supreme Court, the New York Court of Appeals, and the U.S. Supreme Court, where she has argued nine times. Cases she won before the U.S. Supreme Court include: "Bruesewitz v. Wyeth", "Kiobel v. Royal Dutch Petroleum Co.," and "Granholm v. Heald" (which struck down state prohibition of interstate wine shipping). She also represented Shell Oil in an appeal to limit the company's liability for toxic waste. Sullivan has also filed pro bono briefs in a wide range of civil rights cases, and served as co-counsel for Michael Hardwick in the 1986 landmark gay rights case "Bowers v. Hardwick".