ˈæftɚ ðə dˈɛθ ʌv hɪz fˈɑːðɚɹ ɪn wˈʌn θˈaʊzənd sˈɪkshˈʌndɹɪd sˈɛvənti fˈaɪv, jˈoʊhɑːn ˈɜːnɪst ɪnˈɪʃəli ɡˈʌvɚnd ðə dˈʌtʃi ʌv sˈæksɡˈɑːθəˈɔːlʔn̩bˌɜːɡ, dʒˈɔɪntli wɪð hɪz sˈɪks ˈoʊldɚ bɹˈʌðɚz, æz sˈɛt ˈaʊt ɪn ðɛɹ fˈɑːðɚz wˈɪl.
After the death of his father in 1675, Johann Ernest initially governed the duchy of Saxe-Gotha-Altenburg, jointly with his six older brothers, as set out in their father's will.