Treanor's career in the public sector began as a speechwriter for Secretary of Education Shirley Hufstedler, followed by a clerkship for Judge James L. Oakes of the U.S. Court of Appeals for the Second Circuit. During the Iran-Contra Investigation, Treanor served as associate independent counsel. Later he became deputy assistant attorney general in the Office of Legal Counsel, responsible for supplying advice to the White House and attorney general. Treanor has been cited in Supreme Court opinions twice, including by Chief Justice William Rehnquist in his dissent in "Tahoe-Sierra Preservation Council, Inc. v. Tahoe Regional Planning Agency". He has written in-depth about some of the most controversial and complex turn of the 20th century constitutional law  issues. He examined Congress' authority to define and declare war under the War Powers Clause. He focused on the original understanding of the Founders on the takings clause of the United States Constitution, also known as eminent domain in the United States. He wrote an article chronicling understanding of the scope of constitutional judicial review before Marbury v. Madison. He remains a prolific author of scholarly articles. On June 6, 2006, he debated prominent classical liberal/libertarian law professor and published author Richard Epstein on the topic "Did Progressives Rewrite the Constitution?"