Treffert graduated from the University of Wisconsin Medical School in 1958. He interned in Eugene, Oregon. He completed a residency in psychiatry at University Hospitals (now University of Wisconsin Hospital and Clinics) in Madison, Wisconsin.
He holds a position at Saint Agnes Hospital in Fond du Lac, Wisconsin.
He lives in Fond du Lac, Wisconsin. He is on the staff at Saint Agnes Hospital and serves on the Board of Trustees of Marian University. Treffert was a clinical professor at the University of Wisconsin Medical School.
He is also a clinical professor at University of Wisconsin–Milwaukee.
In 1976, he held positions in the private practice of psychiatry, was executive director of the Fond du Lac County Health Care Center and the medical director of the Alcoholism Rehabilitation Unit of Saint Agnes Hospital in Fond du Lac. From 1979 to 1980, Treffert was president of the State Medical Society of Wisconsin and from 1981 to 1987 was chair of its Board of Directors.
He has also been the president of the Wisconsin Psychiatric Association and the American Association of Psychiatric Administrators. In 1995, he was appointed to the Wisconsin Medical Examining Board and in January 2002 was elected its chairman
Treffert maintains a website on autism, savant syndrome and related conditions hosted by the Wisconsin Medical Society.
Treffert has made appearances on 60 Minutes, Columbia Broadcasting System Evening News, The Philosophy Donahue Show, Discovery Channel, Larry King Live, the Newshour, The Oprah Winfrey Show, Today and in a number of documentaries.
