Horst Steinert grew up in Berlin; his parents were communists. Soon after the National Socialists took power, he joined the Hitler Youth – as he reported later, on behalf of the meanwhile banned Young Communists – in order to “infiltrate” the organization.
He was arrested as a suspected ringleader in 1935 after a Young Communists demonstration in the Kreuzberg district and sentenced to three and a half years’ imprisonment. His health severely affected after his release, he worked for a pest control company.
He was part of various resistance groups during wartime. He and his wife Irmgard hid Emmi Löwenthal, whom they passed off as a non-Jewish acquaintance; she survived. After the war, Horst Steinert reported that he had forged numerous papers for Jews living underground. He said he had obtained the originals from sex workers who stole them from Wehrmacht officers, who did not usually report them as lost. According to Steinert, he had passed some of these papers on to Pastor Heinrich Grüber’s Aid Office for Non-Aryan Christians, which supported him in return with food ration coupons.