He joined the SPD in 1945 and the FDJ in 1946. This part of Germany was occupied by the Soviet military in 1945 and the Soviet Union took a hands-on approach to reconfiguring the political landscape. The Soviets favoured a one-party political structure. The SPD, along with the KPD were forcibly merged into the SED, of which Siegfried Lorenz now became a committed member. In 1946 he was employed in a clerical capacity by the SED party leadership in Annaberg. From 1946 to 1947 he was secretary to the local leadership of the FDJ.
Further education
In 1947 he attended the Economics School at Annaberg, moving on for 1948/49 to the :de:Arbeiter-und-Bauern-Fakultät|ABF at Chemnitz and Leipzig universities. Then he studied Social Sciences at Leipzig University. After this, till 1953, he was the Students' Section Leader with the Central Council of the FDJ. Following a further period of study at the Berlin Region Party School, in 1954 he was appointed Secretary for Agitation and Propaganda for the Berlin Region FDJ, a position he held till 1956. Yet more study followed, this time at the prestigious Karl Marx High School, from which he emerged with a diploma in Social sciences.
Political promotions
In January 1958 he was appointed Second Secretary of the Berlin region FDJ, becoming First Secretary from 1961 till 1968. Additionally, between 1958 and 1967 he was a Berlin city councilor and a candidate-member of the Berlin SED Party leadership.
National politics
From 1961 right through till 1976 Lorenz was a member of the Central Council of the FDJ, where between 1963 and 1967 he represented Berlin. From 1963 till 1990 he was a deputy in the National Legislature where his positions included membership and chairmanship of the Youth Committee. In 1965/66 he held leadership of the party's Media department for the regional Berlin leadership. From 1966 till 1976 he headed up the Youth Department of The Party, while simultaneously holding the position of Youth Commissioner with the Central Committee of the party's Politburo. He was himself a candidate for membership of the Central Committee in 1967, becoming a member of it in 1971. Away from Berlin, between March 1976 and November 1989 Siegfried Lorenz was First Secretary of the SED regional leadership for Chemnitz in the south of the country. In 1985 he gained candidature for the Politburo of the party's central committee, becoming a Politburo member in 1986.