John Forrest Dillon studied both medicine and law at the University of Iowa, and was admitted to the Iowa bar in 1852. He was elected a judgeship in Iowa state court in 1858, and elevated to the Iowa Supreme Court in 1862, before being appointed to a Circuit Court judgeship in 1869. While on the bench, he served as a lecturer at the law school.
After resigning his federal judgeship, he taught law at Columbia Law School and Yale Law School. Judge Dillon left a sizable bequest to the University of Iowa College of Law in 1914, providing scholarships for Iowa law students.