Professor Thorne graduated from City College in New York in 1927 and received his law degree from Harvard Law School in 1932. His academic career continued as a law librarian at Columbia University followed by legal instruction at Northwestern University from 1933 to 1942. During World War II he served in the Navy as a cryptanalyst, an experience that, he later joked, well prepared him for deciphering legal manuscripts. He joined the faculty of the Law School at Yale in 1945, moving to Harvard Law School in 1956 as Professor of Law and History.

After producing his first papers on early law books, Professor Thorne developed a specialty in Roman and Canon law as well as English law. His edition of the early Elizabethan treatise, Upon the Exposicion and Understanding of Statutes, was published in 1942.

Professor Thorne's work on Bracton occupied him for more than 30 years and resulted in a 4-volume translation using a Latin text produced by Professor Woodbine of Yale. During the course of his work, Professor Thorne realized that Bracton was more likely written by authors before Bracton's time, but that he probably had taken over the work at some point. Professor Thorne had planned to publish another volume containing his discoveries, but was never able to do so. Although Professor Thorne's assessment of the actual authorship of Bracton was controversial, his is considered the definitive edition. After his retirement from Harvard Law School in 1980, Professor Thorne continued working and receiving visitors there until his death in 1994.