Hendrik (Henry) Casimir was just seven years old when his father Willem Frederik (William Frederick) died. He succeeded him as stadtholder of Friesland, Groningen and Drenthe. His mother Albertine Agnes acted as regent for her young son.
Besides being stadtholder Hendrik Casimir also held an important position in the army of the Republic of the United Netherlands. Relations with his cousin, the commander-in-chief and King-Stadtholder Willem III of Orange were not good, however.
Hendrik Casimir married his cousin Henriette Amalia of Anhalt-Dessau, one of Frederik Hendrik’s (Frederick Henry) granddaughters. They had nine children, but one of their sons died at an early age. After his death Hendrik Casimir was succeeded as stadtholder by his son Johan Willem Friso (John William Friso).