He has made extensive contributions to the study of the Olmecs' civilization, which flourished in the Gulf Coast of Mexico region during the pre-classic period in Mesoamerica.
He then pursued graduate studies in anthropology at Penn State, earning his MA under the supervision of archaeologist William T. Sanders in 1965.
[4] 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.
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.