Lachmann was born in Liegnitz on November 6, 1909. He began his career as a policeman. From September 1941, at Trawniki concentration camp Lachmann trained Ukrainians who had volunteered to be guards at the Reinhard death camps. According to Lachmann's own statement, he was in Sobibor as commander of the Ukrainian guards since August 1943. However, witnesses state that he was in the camp starting exactly one year earlier. Fellow SS man Erich Bauer called him "a boozer and somebody who stole like the ravens". Sobibor prisoners such as Eda Lichtman and Abraham Margulies witnessed him rape young girls.
When Franz Reichleitner took over command of Sobibor from Franz Stangl, he sent Lachmann back to Trawniki because he deemed that Lachmann was unfit for duty. From there Lachmann deserted with his Polish girlfriend in the winter of 1942-43. He was arrested several months later in Warsaw and sentenced by an SS and police court to six years in prison. However, he was released in April 1945 during the final stages of the war, captured by the Soviet Red Army and survived the war.
In the so-called "Sobibor Trial" in Hagen, which lasted from September 6, 1965 to December 20, 1966, he was accused of participating in the mass murder of approximately 150,000 Jews. Lachmann is quoted as saying: "I had nothing against the Jews. I regarded them as all other people. My suits I previously bought from a Jew, Max Süssmann, who had a textile firm in Liegnitz." The court found Lachmann to be mentally incompetent and he was acquitted because of "putative duress".