Klaus Siebert won the silver medal with the biathlon relay at the 1980 Lake Placid Winter Olympics together with Frank Ullrich, Mathias Jung and Eberhard Rösch, finishing behind the Soviet Union. Siebert also placed fourth in the 10 km and 15th in the 20 km. At the World Championships he won three gold (1978-79 – relay, 1979 – 20 km) and three bronze medals (1977 – relay, 1975/78 –10 km). Domestically he added three titles in the 10 km (1977, 1978, 1980), and a third place in 1976. He also claimed two more titles over 20 km in 1978 and 1979, and was runner-up in 1977 and 1980. With the SG Dynamo Zinnwald relay team Siebert won six titles in a row (1975-80). In 1979 he also won the overall World Cup.
Even during his active career he started coaching. He became one of the most successful biathlon coaches ever, first coaching the youth national team of GDR from 1984 to German reunification, then up to 2002 as an assistant coach of the German national team, also coaching individually biathletes like Ricco Groß, Katja Beer and Carsten Heymann. He then took over the national teams from Austria (2002-06), China (2006-08) and finally Belarus (2008-14), bringing Darya Domracheva to the top of the world. In 2010 he had severe health problems with colon cancer, but won this fight as well.