Returning to his native Korea, he began studies of the staple Korean diet and its effects on metabolism as a research assistant at Kyŏngsŏng Medical College in February 1928.  He was appointed an instructor of physiology in the department of biochemistry of Severance Union Medical College (now Yonsei University College of Medicine) and an adjunct instructor of dietetics at Ewha Womans University College of Medicine.
In 1932, Suksin Lee was the first Korean to earn a Ph.D. in biochemistry for his thesis, A Study on the Eating Habits of Koreans, presented to Kyoto Imperial University on the nutrition and metabolism of prisoners in Korea. Among his advisers at the time was Professor Sato of Keijo Imperial University.
He was then appointed full-time professor of biochemistry in 1933 at Severance Union Medical College, the first Korean to hold such a position.  He continued to lead the department, later serving as Severance's Dean of Student Affairs, until his death aged approximately 47 of a cerebral hemorrhage on 12 December 1944.