Immediately after law school, Farnsworth served as a law clerk for Judge Richard A. Posner on the Seventh Circuit and then clerked for Justice Anthony M. Kennedy of the United States Supreme Court.

He then worked as a Legal Adviser to the Iran-United States Claims Tribunal in The Hague.

Prior to beginning his tenure as dean at Texas, Farnsworth taught for 15 years at Boston University School of Law,[2] and served several years as associate dean for academic affairs.

He is author of Restitution: Civil Liability for Unjust Enrichment (Chicago 2014),[4] and The Legal Analyst: A Toolkit for Thinking About the Law (Chicago 2007),[5] an acclaimed guide to interdisciplinary approaches to legal thought.

He is Reporter for the American Law Institute’s Restatement Third, Torts: Liability for Economic Harm.