He served as the Regent of the Duchy of Saxe-Coburg and Gotha during the minority of his wife's cousin, Duke Charles Edward, from 1900 to 1905.

After finishing high school in Karlsruhe, the young prince studied law in Paris, Bonn, Tübingen and Leipzig, where he graduated in 1885 with the first legal exam in Naumburg.

[2] From 1894 to 1897, he was detached to work in Strasbourg as an assistant to his father, Prince Hermann, who was the Imperial Governor of Alsace-Lorraine.

[3] During the First World War, the Prince was active in voluntary military medical aid efforts (freiwillige Krankenpflege).

[4] From 19 July 1915 to 5 October 1915, he was sent as a special envoy to the German Embassy in Constantinople and to the Balkans to assume the duties of the envoy Hans Freiherr von Wangenheim who had fallen ill.[2] He later served as the General Delegate to the staff of the Supreme Commander East (Oberbefehlshaber Ost) on the Eastern Front and on 10 July 1918 became the Imperial Commissioner and Military Inspector of Voluntary Medical Aid (Kaiserlicher Kommissar und Militärinspekteur der freiwilligen Krankenpflege).

[5][6] After Adolf Hitler came to power in 1933, Ernst joined his son (who had already entered in 1931) in the Nazi Party with the membership number of 3726902.

Ernst was dedicated to church and nursing activities and was a member of the German Evangelical Church Assembly (the Kirchentag), Commander of the Württemberg-Badenschen Genossenschaft (Württemberg-Baden Cooperative), Governor of the Bailiwick of Brandenburg Order of St. John, Honorary President of the Württemberg State Association of the Red Cross as well as of the Evangelical People's League of Württemberg (Evangelischen Volksbund für Württemberg).