Lach Mean was recruited as a Khmer Rouge militia member in 1974 before becoming a soldier. In 1975, he was sent for training at the military technical school at Ta Khmau. He was then assigned to work as a guard at Ta Khmau prison and later on at Dam Pheng prison in Phnom Penh. When this prison was relocated to S-21, he worked there as a guard and typist. In late 1978 he as transferred to the interrogation unit where he received training under the supervision of Kak.
As a witness, Lach Mean described the working conditions at S-21, reporting that S-21 staff disappeared on a regular basis. He described the detention conditions as well as the interrogation sessions, which aimed to extract information from the prisoners and identify their networks. His training to become an interrogator consisted of observing his supervisor interrogate detainees. He told the Trial Chamber that he was only assigned to interrogate ordinary detainees and was not permitted to inflict torture upon them. He acknowledged before the Chamber that he personally interrogated only three to four detainees. He testified on the torture methods used at S-21, such as beating prisoners with a guava tree stick and applying electroshocks to their ears. As an interrogator at S-21, he recalled meeting the Accused on a daily basis.