Ernst Johann started his career in Jelgava, at the court of the widow of Duke Friedrich Wilhelm. Having become her favourite, he exerted strong influence on the administration of the Duchy for nearly twenty years. In 1730 Anna Ioannovna became Empress of Russia and granted Biron the title of Count and senior chamberlain’s position. After the death of the last duke from the Kettler dynasty, he was elected Duke of Courland. Two ruler’s residences of European level were built for him in Rundāle and Jelgava. After the death of Anna Ioannovna, Ernst Johann was appointed Regent of Russia , but in 22 days he was arrested and sentenced to death, later he was pardoned and exiled. In 1762 Emperor Peter III of Russia restored Biron’s titles, but Empress Catherine II allowed him to return to Courland.
Ernst Johann was buried in the vault on January 2, 1773 in a luxurious wooden coffin. The copper sarcophagus was made in 1784 by craftsmen from Danzig J. D. Peppich and G. Kaufmann. On the lid, there is a gilded copper plaque with the coat of arms of Courland-Semigallia. The burial was destroyed in 1919 and after World War II. Restoration of the sarcophagus was finished in 1990 and the burial was consecrated.