Cahill started working as a medical assistant at the Biochemical Department at the Peter Bent Bringham Hospital (today:
Brigham and Women’s Hospital) in Boston. He was also involved in research at the Albert Baird Hastings laboratory from 1955 until 1957 before he transferred to the Joslin's Diabetes Center in 1958, after another clinical year. There, he took the place of research director when Albert Renold returned to Europe 1962. Cahill held the position until 1978. 1962 he became active in the research at the
Howard Hughes Medical Institute (HHMI), since 1972 as member of the advisory board and later as research director and finally vice president. He left the HHMI in 1990. 1970 he became a
Harvard Medical School professor. Until 1990 he continued teaching classes, before he had been given emeritus status. Since 1989 Cahill taught a biology course for non-biologists at the
Dartmouth College. == Personal life ==