Lillie is a member of the International Arbitration Group at White & Case. She represents and advises foreign sovereigns and private companies in commercial and investor-state arbitrations, including before tribunals constituted under the International Centre for Settlement of Investment Disputes (ICSID) and the International Chamber of Commerce (ICC). She has an active pro bono practice focusing on immigration matters and serves on the D.C. Responsible Business Committee.

Prior to joining White & Case, Lillie completed a Fulbright Award in Madrid, Spain. As a Monroe Leigh Fellow in International Law, she interned at an international human rights advocacy organization in Costa Rica.

At the University of Virginia School of Law, Lillie served as a Notes Editor on the Virginia Journal of International Law, participated in the International Human Rights Law Clinic, and presented research on international investment law as a Salzburg Cutler Fellow. She also holds a Master's in Economic Law from Sciences Po Law School (Paris) and a Master's in Migration Studies from Pompeu Fabra University (Barcelona).