His 59 - year reign is the longest of Danish monarchs .
Christian IV of Denmark ( 1577 -- 1648 ) , king of Denmark - Norway and Duke of Holstein and Schleswig from 1588 to 1648 .
king 's 59 - year reign is the longest of Danish monarchs .
Christian IV of Denmark ( 1577 -- 1648 ) , king of Denmark - Norway and Duke of Holstein and Schleswig from 1588 to 1648 .