Diehl was born in
Bethlehem, Pennsylvania, United States, in 1940. He attended
Penn State University, where he graduated with a
BA degree in history. He then pursued graduate studies in anthropology at Penn State, earning his
MA under the supervision of archaeologist
William T. Sanders in 1965. His thesis was
The Use of Ethnographic Data for Archaeological Interpretation of the Teotihuacan Valley, Mexico. In 1969, he received his
PhD; his doctoral thesis was
An Evaluation of Cultural Evolution in the Formative Period in Mesoamerican Prehistory, again with Sanders as his supervisor. Diehl's experiences in archaeological fieldwork began as an undergraduate at Penn State. From 1961 to 1964, he participated in field trips to the
Valley of Mexico, the archaeological locality in the
central Mexican altiplano, where the major city and
polity of
Teotihuacan reached its apogee during the Classic era. Working under the tutelage of Sanders, Diehl conducted research at Teotihuacan and other nearby sites, gaining experience in
excavation techniques,
archaeological field surveys and
ethnography. As a graduate working towards his doctorate, Diehl worked with
Yale University professor
Michael D. Coe in the 1966–1967 field season at
San Lorenzo Tenochtitlán, a major
Olmec site in the Mexican state of
Veracruz. ==Career==