Karel Louis Leons was the youngest of the two children of Salomon Leons and Dina Selina Wolf. The eldest child was his sister Esther Jeannette, born in 1905, and who was married to Leendert Bleekrode in Den Haag on 22 November 1933. Both have survived the Holocaust. (Leendert's grandfather was Mozes Bleekrode, born 4 July 1862 and died at Camp Westerbork 1 April 1943).
Karel Louis married Elise Philips van Buren in Amsterdam on 28 February 1935, a daughter of Frederik Philips van Buren en Nanny Herz. The couple had no children and have been divorced on 20 Maart 1939 in Amsterdam. Karel Louis married again to Catharina Groen, a daughter of Eliazer Groen en Rosina de Haan.
Karel Louis Leons has been deported on 18 September 1942 from transit camp Westerbork to Auschwitz, where he eventually lost his life on 31 December 1942. His wife Catharine has been sent too to Auschwitz on 18 September 1942 but she has been killed there immediately upon arrival there on 21 September 1942.
Municipal Archive of Den Haag, family registration cards of Salomon Leons and Eliazer Groen; certificate of death C3764 dated 17 November 1950 of Karel Louis Leons; City Archive of Rotterdam, index marriages; City Archive of Amsterdam, archive card of Mozes Bleekrode and the file cabinet of the Jewish Council of Amsterdam, records of Karel Louis Leons and Catharina Leons-Groen.