It’s most common to find professors teaching with only a master’s degree at the community college level, or working as adjunct faculty at four-year colleges. Adjuncts are the academic version of temps, but they make up the majority of faculty in American universities. According to NCES, in 2018, 54 percent of instructors at degree-granting post-secondary institutions were adjunct faculty.
In many fields, however, there is a surplus of PhD graduates, which makes competition stiff even for adjunct positions. In these fields, a master’s-level professor position is rare.