Harndt studied medicine and dentistry in Berlin from 1920 to 1926 and went on to work at the Institute of Dentistry at Friedrich-Wilhelms-Universität starting in 1927.

Harndt became a member of Germany’s Nazi party in 1936. Later he would claim that his appointment to a lecturer position in 1938 was delayed for two years due to his “political unreliability.” A historical analysis of his claims during denazification, however, reveals contradictions. In 2023, medical historian Dominik Groß came to the conclusion that Harndt should not be considered a victim of the Nazi regime. Instead, he was one of their political proponents. While he might not have been a Nazi out of strong personal conviction, he served the regime as a member of various Nazi organizations. He was also involved in Nazi networks and supported the national socialist “health” policies. For a more extensive discussion of Harndt’s relationship to the Nazis see Groß, Dominik: “Ein komplexer Fall: Ewald Harndt (1901-1996) und sein Verhältnis zum Nationalsozialismus”; Deutsche Zahnärztliche Zeitschrift 2/2023, KVM - Der Medizinverlag, 2023.

In December 1945, the Soviet military administration fired Harndt from his position at the university due to his membership in the Nazi Party. But just one year later he would return to serve as the interim director of the department of restorative dentistry at the Institute of Dentistry, where he would receive a professorship in 1948. In May 1950, Harndt became the director of the Institute of Dentistry at Humboldt-Universität (formerly Berlin University). In November of that same year, he resigned due to political reasons and switched to Freie Universität Berlin in 1951.

In October 1956, Harndt became a professor of dentistry and oral and maxillofacial surgery at Freie Universität and took over the leadership of the Polyclinic for Dentistry and Oral and Maxillofacial Surgery.

From 1967 to 1969 he served as Freie Universität Berlin’s twelfth rector.

Harndt was named an honorary member of a large number of dental societies and associations and received many international awards and distinctions. He is also known outside of his professional field for his book Französisch im Berliner Jargon (“French in Berlin Slang”) that looks at how the Huguenots influenced colloquial language in Berlin. It was first published in 1977 and has remained in print ever since. It is now in its fifth edition.

In 2001, the first Ewald Harndt Symposium was held in honor of Harndt’s 100th birthday. At the event, the Berlin Association of Dentists introduced the Ewald Harndt Medal and awarded it to a group of individuals who devoted themselves to providing dental care to the unhoused. In 2019 the name of the award was changed to the “Philipp Pfaff Prize” in response to the revelations surrounding Harndt’s Nazi past.