In 1933, after the Nazis had seized power, he was working illegally for the KPD and ended up in "protective custody" many times. From October 1933 until 1936, he was at Börgermoor and Esterwegen concentration camps, and until 1937 in pre-trial custody in Duisburg. He was sentenced by the Volksgerichtshof to one year and nine months in prison, released from custody, and banished from the Ruhr area.
In April, Lemmnitz emigrated to the Netherlands, where he became a member of the KPD leadership in Amsterdam. After the Wehrmacht marched into the Netherlands, he was once again arrested and then sentenced by the Volksgerichtshof to ten years in labour prison (Zuchthaus). Until 1945, he was an inmate at the labour prison in Brandenburg-Görden.
In 1946, Lemmnitz joined the Socialist Unity Party of Germany (SED), took his studies up once again, graduating from the University of Leipzig in 1948. From 1948 to 1953, he held the chair for political economy at the Party College, and then until 1955, he was Professor of Political Economy and Dean of the Faculty of Economics at the University of Rostock. Until 1956, he was Rector of the College for Financial Science in Potsdam-Babelsberg, and until 1958, he held the same position at the Berlin College for Economics.
In December 1958, he succeeded Fritz Lange as Minister for National Education, thereby also becoming a member of the Council of Ministers of the GDR and the Ideological Commission at the SED Central Committee's Politburo. In November 1963, he was succeeded by Margot Honecker, who would hold the post until 1989. From 1963 to 1965, he was a scientific associate at the Economic Institute of the Academy of Sciences, until 1971, Acting Director of the German Economic Institute, and finally Associate of the Institute for International Politics and Science.
