After the death of his father, in 1691, Frederick II assumed the duchy of Saxe-Gotha-Altenburg.
Because he was still under age, a guardianship and co-regency was formed between his uncles, the dukes Bernhard I of Saxe-Meiningen and Heinrich of Saxe-Römhild. In 1693, after he returned from a journey to Holland and England, he wrote to the emperor for a license of adult age and took independent control of the government of his duchy. Frederick was a splendor-loving baroque ruler; his yard attitude and the standing army, whom he had taken over from his father and even expanded, devoured some of his income. As a solution, Frederick rented his soldiers to foreign princes, which brought him a great difficulty in 1702, when the King Louis XIV of France rented his troops and used them in his war against the Emperor.
Relating to domestic affairs, Frederick essentially continued the policy of his father. He created an orphanage in Altenburg (1715), a breed and a lunatic asylum in Kahla (1726), as well as the Magdalenenstift -in honor of his mother and wife (both with the same name)- (1705), a school for noble women. For 100,000 talents from his private property, he bought the famous numismatic collection of the Prince Anton Günther of Schwarzburg Arnstadt, which formed the current collection of coins (Münzkabinetts) in the Schloss Friedenstein.
By accumulation of parts of Saxe-Coburg (became extinct in 1699), Saxe-Eisenberg (became extinct in 1707) and Saxe-Römhild (became extinct in 1710), he succeeded to all, however only at long hereditary disputes under the other Ernestine Duchies, which went only to 1735 with an arbitral award of the Emperor finally to end reaching in each case area increases for his country.
Wilhelm (b. Gotha, 12 March 1701 - d. Gräfentonna, 31 May 1771), married on 8 November 1742 to Anna of Holstein-Gottorp. Their marriage was childless.
Karl Frederick (b. Gotha, 20 September 1702 - d. [of smallpox?] Gotha, 21 November 1703).
a daughter (b. and d. Gotha, 8 May 1703).
Johann August (b. Gotha, 17 February 1704 - d. Stadtroda, 8 May 1767).
Christian (b. Gotha, 27 February 1705 - d. of smallpox, Gotha, 5 March 1705).
Christian Wilhelm (b. Gotha, 28 May 1706 - d. Stadtroda, 19 July 1748), married on 27 May 1743 to Luise Reuss of Schleiz. Their marriage was childless.
Ludwig Ernst (b. Gotha, 28 December 1707 - d. Gotha, 13 August 1763).
Emanuel (b. Gotha, 5 April 1709 - d. Gotha, 10 October 1710).
Moritz (b. Altenburg, 11 May 1711 - d. Altenburg, 3 September 1777).
Sophie (b. Altenburg, 23 August 1712 - d. Altenburg, 12 November 1712).
Karl (b. Gotha, 17 April 1714 - d. Gotha, 10 July 1715).