Frederick Christian I, Duke of Schleswig-Holstein-Sonderburg-Augustenburg

He was born into the Augustenburg branch of the House of Oldenburg, as the eldest son of Christian August, Duke of Schleswig-Holstein-Sonderburg-Augustenburg (1696–1754) and Duchess Louise Frederikke née Countess of Danneskiold-Samsøe (1699–1744).

In 1754, his father died and Frederick Christian inherited Augustenborg Castle and Gråsten. However, these estates were deeply in debt. He waived his claims on the duchies of Schleswig and Holstein and in return the King of Denmark (who was also Duke of Schleswig and Holstein) granted him a favourable settlement. This allowed him to purchase Als and Sundeved, making him the largest landowner in Schleswig. He was also able to expand Augustenborg Castle, the family residence.

Duke Frederick Christian served as a general in the Danish army. He was also made a Knight of the Order of the Elephant.

On 26 May 1762, Frederick Christian married Princess Charlotte Amalie Wilhelmine of Schleswig-Holstein-Sonderburg-Plön (1744–1770), the youngest but one daughter of Frederick Charles, Duke of Schleswig-Holstein-Sonderburg-Plön and Countess Christine Armgard von Reventlow (1711–1779), niece of the Danish queen consort Anne Sophie von Reventlow.

The couple had seven children:

• Louise Christine Caroline (16 February 1763 – 27 January 1764);
• Louise Christine Caroline (17 February 1764 – 2 August 1815);
• Frederick Christian II (28 September 1765 – 14 June 1814), married Princess Louise Auguste of Denmark;
• Frederick Charles Emil (8 March 1767 – 14 June 1841), Danish General, married Sofie Eleonore Friederike of Scheel (1776–1836); daughter of Jürgen Eric of Scheel;
• Christian August (9 July 1768 – 28 May 1810), Danish general and, later, Crown Prince of Sweden as Karl August (died before he could ascend the throne);
• Sophie Amelie (10 August 1769 – 6 October 1769);
• Charles William (4 October 1770 – 22 February 1771).