After an offer from Professor William Prager, Lee and his family returned to the United States in 1948, where he was a Professor of Applied Mathematics at Brown University for 14 years. He served as Chairman of the Applied Mathematics Division for five years. During these years, faculty members in the Divisions of Applied Mathematics and Engineering, which included Dan Drucker, Harry Kolsky, Allen Pipkin, Paul Symonds, Ronald Rivlin, Dick Shield, and Eli Sternberg in addition to Prager and Lee, made Brown the worldwide center for research in solid mechanics. In 1962, Ras was appointed as a Professor in the Division of Applied Mechanics and the Department of Aeronautics and Astronautics at Stanford University, joining Norman Goodier, Wilhelm Flügge, Nick Hoff, and Miklos Hetenyi in the widely acclaimed Stanford applied mechanics group. Almost every graduate student in solid mechanics during that time took Lee's sequence of three courses (each two quarters long) in nonlinear continuum mechanics, viscoelasticity, and plasticity. He remained at Stanford for 20 years (1962–1982), taking mandatory retirement at the age of 65. For the last 10 years of his professional career, Ras was the Rosalind and John J. Redfern, Jr. Chair Professor of Engineering at Rensselaer Polytechnic Institute.