Leslie Brent was born Lothar Baruch on 5 July 1925 in Köslin, Germany (Koszalin, Poland). When the discrimination against Jewish children was made official, and after experiencing antisemitism at school, Leslie was sent to the Jewish Orphanage in Pankow, Berlin. He describes the years in the orphanage as one of the more traumatic experiences in his life, coming from a small protective family to a big institution.

He was selected by the director of the orphanage for the first Kindertransport and arrived at Dovercourt Camp on 2 December 1938. Leslie spent three weeks in the camp before going to Bunce Court, a German-Jewish boarding school which was run by Anna Essinger. He was young enough to avoid internment, but after being classified as an enemy alien, he joined the British forces 1944-1947. After the army he studied Zoology at Birmingham University, and was accepted as a postgraduate student at University College London under Peter Medawar (who was awarded a Nobel Prize).