 Through his mother he was descendant of King William II of the Netherlands

In 1935 he was adopted by one of his relatives, Heinrich XLV, Hereditary Prince Reuss Younger Line (1895–1945) member of the House of Reuss Younger Line, the adoption took place for inheritance reasons, but not for succession rights for the headship of the House of Reuss, instead Prince Heinrich IV succeeded as "Prince Reuss".Heinrich I married 15 September 1939 at Bad Doberan to Duchess Woizlawa Feodora of Mecklenburg-Schwerin (1918–2019), daughter of Duke Adolf Friedrich of Mecklenburg-Schwerin, and his wife, Princess Victoria Feodora Reuss of Schleiz.His wife was the niece of his adoptive father Heinrich XLV.