He was an assistant professor of pathology and chair of the department of dermatopathology at Harvard University and Massachusetts General Hospital from 1962 until 1969.
He conducted research, primarily at Beth Israel Hospital, until just a few days before his death in November 1997.
From these studies he developed a five-part scale, based on the depth of penetration of the lesion from the epidermis into the dermis and down to the subcutaneous tissue, which can be used to predict the likely progression of the tumor and the prognosis for the patient.
[5] While largely superseded by other prognostic attributes, as of 2015[update] pathologists evaluating a melanoma still classify it according to its Clark's level, and treating physicians consider that information to choose the appropriate treatment.
[1] He was a strong a proponent of patient education and gave many public lectures teaching people how to recognize the warning signs of a possible melanoma.
[8] The University of Pennsylvania School of Medicine hosts an annual Wallace H. Clark Jr., MD, Lectureship in Cutaneous Oncology as a living memorial to Clark, describing him as a "caring and expert physician, an imaginative and rigorous investigator, and a charismatic and provocative teacher," whose "recognition of what makes people susceptible to melanoma and the appearance of early, highly curable forms of melanoma has saved countless lives.