Dr. Klein joined the faculty of the university’s Department of History in 1969. He was designated an Alumni Distinguished Service Professor in 1977 and Lindsey Young Professor in 1980. He retired from the university in 1984 and was appointed part-time UT Knoxville historian in 1988, a position he held until the office was abolished in 1997. As part of the institution’s bicentennial celebration, a collection of vignettes the Office of the University Historian had published in the in-house newspaper Context were printed in monograph form.
A specialist in early American history, Klein authored The Politics of Diversity: Essays in the History of Colonial New York (1974) and edited New York: The Centennial Years, 1676–1976 (1976) and Law in Early New York (1978). He additionally wrote many articles for scholarly journals. He held a Ford Foundation Fellowship (1955–56) and a Fulbright Fellowship to New Zealand (1962). The UT Alumni Association, Mortar Board, and the Society of Daughters of Colonial Wars recognized him as an outstanding teacher. He was elected a member of the American Antiquarian Society (1986) and of the Massachusetts Historical Society (1989). He served as president of the American Society for Legal History from 1980 until 1982.
His widow, Margaret, created the Milton M. Klein History Studies Endowment, which supports a visiting-scholar lecture series in early American history. His intellectual contributions to the life of the university have been recognized by placement of a plaque in his honor on a faculty study.