He first worked as a bandmaster at the Freiberg Theatre and as musical director and conductor of the symphony concerts at the Brandenburg/H Stadttheater.
In 1959, Kleinert was appointed General Music Director in the GDR and took over as chief conductor of the Berlin Radio Symphony Orchestra.
Through his intensive commitment in the search for suitable musicians, he succeeded in making the orchestra playable again and preserving its specific sound.
Concert tours with the orchestra took Kleinert to Italy, England, West Germany, the Soviet Union, Poland and Bulgaria.
An excellent percussion technique, a great sense of sound and an absolute ear were Kleinert's essential qualities.