[1][2] Lachmann was born in Beijing, China to mother Ingeborg Møller (1889–1963) and father, Vilhelm Petersen (1877–1964).

In their new family, the three sisters were given the opportunity to practice different sports, and Karen chose fencing, which had become an option for women from around the turn of the twentieth century.

[3] Karen Lachmann participated in four Olympic Games: 1936 in Berlin, 1948 in London, 1952 in Helsinki and 1956 in Melbourne.

At all four, she reached the final round with the following results: In 1936 she achieved fifth place, in 1948 she won a silver medal, in 1952 bronze and in 1956, she came in sixth.

[3] Lachmann was trained as a secretary and was employed for many years at the University of Copenhagen's Department of Pharmacology.