After completing law school and graduate school, Orth clerked for Judge John J. Gibbons of the United States Court of Appeals for the Third Circuit. He is the author of The Judicial Power of the United States: The Eleventh Amendment in American History (1987), Combination and Conspiracy: The Legal History of Trade Unionism, 1721-1906 (1991), The North Carolina State Constitution: A Reference Guide (1993), Due Process of Law: A Brief History (2003), and How Many Judges Does it Take to Make a Supreme Court? and Other Essays on Law and the Constitution (2006), as well as of numerous scholarly articles and book reviews. He contributes the chapters on concurrent estates to Thompson on Real Property. He was an associate editor (for law) of the American National Biography and contributed to that series, as well as to the Oxford Companion to the Supreme Court of the United States, the Oxford Companion to American Law, the Oxford International Encyclopedia of Legal History, the Yale Dictionary of Legal Biography, as well as many others. He reviews books for numerous scholarly publications, including the American Historical Review, and the Journal of American History. His publications have been cited by federal and state courts, including the United States Supreme Court and the North Carolina Supreme Court. In a recent North Carolina redistricting case he was referred to as "a highly respected state constitutional scholar." He joined the UNC-Chapel Hill faculty in 1978 and teaches basic and advanced property and legal history.