Casimir William was the youngest son of Landgrave Frederick II of Hesse-Homburg (1633–1708), the famous Prince of Homburg, from his second marriage with Louise Elisabeth (1646–1690), the daughter of the Duke Jacob of Courland.

He was educated together with his three years younger half-brother George Louis (from Frederick II's his third marriage with Countess Sophie Sybille of Leiningen-Westerburg (1656–1724).

During a visit to his cousin Duke Frederick William of Mecklenburg-Schwerin the passion for hunting, which he shared with his father and his brothers, woke up in him.

Since his older brother Frederick III and his two sons preceded him in the line of succession, he opted for a military career and fought in 1708 in a Mecklenburg regiment under Prince Eugene of Savoy.

He married on 9 October 1722 in Braunfels with Charlotte Christine (1690–1751), a daughter of Count William Maurice of Solms-Braunfels.