Ernst was the second son of Johann Casimir, Duke of Saxe-Coburg, and his wife, Princess Margaret of Brunswick-Lüneburg. Ernst was a member of the Ernestine branch of the House of Wettin, which ruled over various territories in Thuringia.
Ernst received a comprehensive education in various subjects, including history, philosophy, and theology. He also developed a strong interest in music and was a skilled composer himself. In 1630, after his elder brother's death, Ernst inherited the Duchy of Saxe-Gotha from his father, while his younger brother inherited the Duchy of Saxe-Altenburg.
As a ruler, Ernst implemented a number of reforms that improved the governance and economic situation of his duchy. He also supported the arts and sciences and established a court orchestra, which included renowned musicians such as Johann Sebastian Bach and Georg Philipp Telemann.
Ernst was known for his piety and devotion to the Lutheran faith, and he was actively involved in church affairs. He also had a strong interest in alchemy and supported alchemists in his court.
Ernst died on 26 March 1675, in Gotha. He was succeeded by his son, Frederick I.