I am trying to understand when cognitive dissonance was first defined. Would mental health professionals have known about the concept of cognitive dissonance in 1972?
Glen McGhee's "A Cultural History of Cognitive Dissonance," (2005/2014) notes that Gustav Ichheiser (1897-1969) dealt with personal dissonance and dissonance reduction in 1928 (212 Note 2). Also Kurt Lewin's tension theory, and Festinger's level of aspiration work in 1940.
Lorne L. Dawson also has an interesting chapter, "Clearing the Underbrush: Moving Beyond Festinger To A New Paradigm For The Study of Failed Prophecy" (2011) that reminds us of the seminal role that Festinger/Riecken/Schachter, "When Prophecy Fails" (1956) plays in the development of cognitive dissonance theory.
Cognitive dissonance has been a part of social psychology and experimental psychology for a long time -- but what do you mean by "mental health professionals"? How were they trained? Where did they go to school? In the 1970s, there were numerous therapies, including Gestalt, Rogerian, Reichian, Freudian, Jungian, etc. Or are you talking about social workers? MSWs?