Following his graduation, from 1925 to 1929 Niles served as a debate coach and English professor at the University of Colorado.
Upon receiving his L.L.M., Niles joined the faculty of New York University School of Law, where he became a full professor in 1944.
From 1942 to 1944, Niles was also a Trusts and Estates lawyer at the New York firm Putnam & Roberts.
After retiring, Niles taught as an emeritus professor at University of California, Hastings College of the Law.
He chaired the American Bar Association's Section on Real Property Probate and Trust Law; served as director of the Institute of Judicial Administration; and from 1966 to 1967 served as president of the New York City Bar Association.