Professor Nielson lectures and writes in the areas of administrative law, civil procedure, and federal courts. His publications have appeared in journals such as the University of Pennsylvania Law Review, Duke Law Journal, University of Chicago Law Review, Northwestern University Law Review, and Georgetown Law Journal. He serves as Chair of the Administrative and Management Committee of the Administrative Conference of the United States, a federal agency that studies the administrative process and makes recommendations on ways to improve it. He also serves on the Council of the American Bar Association’s Section of Administrative Law & Regulatory Practice. Previously he chaired or co-chaired the Section's Rulemaking and Antitrust & Trade Regulation Committees. Professor Nielson is a permanent commentator at the Yale Journal on Regulation's Notice & Comment blog where he focuses on the U.S. Court of Appeals for the District of Columbia Circuit. He has been quoted regarding the D.C. Circuit in the Wall Street Journal, the Washington Post, Bloomberg BNA, the National Law Journal, and Law360.

Professor Nielson was appointed by the U.S. Supreme Court to brief and argue Collins v. Yellen, a separation-of-powers case about the Federal Housing Finance Agency.

Before joining the faculty, Professor Nielson was a partner in the Washington, D.C. office of Kirkland & Ellis LLP. He also has served as a law clerk to Justice Samuel A. Alito, Jr. of the U.S. Supreme Court, Judge Janice Rogers Brown of the U.S. Court of Appeals for the D.C. Circuit, and Judge Jerry E. Smith of the U.S. Court of Appeals for the Fifth Circuit.

Professor Nielson received his J.D. from Harvard Law School. Following graduation, he was awarded a Harvard Law School Post-Graduate Research Fellowship. Professor Nielson also received an LL.M from the University of Cambridge, where he focused his studies on the institutions that regulate global competition and commerce. He received his undergraduate degree from the University of Pennsylvania, majoring in economics and political science.