She continued to Yale University for her MA (1959) and PhD (1965), studying there with historians of Christianity Roland Bainton and Jaroslav Pelikan.
In the 1960s she led the campaign to abolish a nepotism rule at Oberlin, which prevented both members of a couple from serving on the faculty.
[3] She used a Guggenheim Fellowship in 1989–1990 to work on her book on Peter Lombard, for which she won the Haskins Medal.
[1] In the early 1990s she helped lead the campaign to reform Oberlin's sexual harassment policy.
[3] On retirement from Oberlin, Colish moved to Guilford to be nearer to Long Island Sound and Yale University.