Karl was the eldest son of Friedrich Wilhelm, Duke of Schleswig-Holstein-Sonderburg-Glücksburg, and Princess Louise Caroline of Hesse-Kassel and an elder brother of Christian IX of Denmark.

Upon his father's death in 1831, Karl inherited Glücksburg Castle and became Duke of Schleswig-Holstein-Sonderburg-Glücksburg at the age of seventeen.

[3] Initially, the young prince grew up with his parents and many brothers and sisters at the miniature court of his maternal grandparents at Gottorf Castle, the habitual seat of the royal governors of the duchies of Schleswig and Holstein.

[4] Subsequently, the family moved to Glücksburg Castle, where Prince Karl was raised with his siblings under their father's supervision.

In 1831, King Frederick VI appointed him staff captain in the Oldenburg Infantry Regiment which was stationed in Rendsburg and headed by his father.

In 1846, as head of the House of Glücksburg, he protested against King Christian VIII's open letter on the succession in the duchies of Schleswig and Holstein, and out of dissatisfaction resigned from the Army in August of the same year.

[6] Duke Karl actively sided against Denmark during the First Schleswig War from 1848 to 1851, which caused the ducal couple's relations with the Danish Royal family to be severed.

During the war, he initially took command of one of the provisional Schleswig-Holstein government's Infantry Brigades, but already in the autumn of 1848, he and his wife moved to Dresden in the Kingdom of Saxony.