From 1962 to 1980, Westwater was head of the Department of Chemical Engineering at Illinois. During that time there was a substantial increase in scholarly contributions and significant expansion of the undergraduate and graduate programs. He was known on the Illinois campus as a skillful administrator and a dedicated teacher who had great empathy for the needs and desires of undergraduates. He impressed upon his many students the importance of a sound understanding of the physical fundamentals of chemical engineering processes as a basis for the development and application of those processes. A professorship in his honor was established by friends and admirers in 1986. He retired in 1988.

James traveled extensively as an invited lecturer and consultant to many foreign countries, including Communist Russia, China, India, and Japan. He and his wife traveled, for business and pleasure, to every continent. Other interests included gardening, reading, swimming, and handball.

His wife, Elizabeth Keener Westwater, died on July 17, 2007. He is survived by four children, Barbara Westwater, David Westwater, Judith White, and Beverly Moore; seven grandchildren; seven great-grandchildren; and a brother, Robert Westwater.