He was the eldest son of the Prince Hermann of Saxe-Weimar-Eisenach and his wife, Princess Augusta of Württemberg (1826-1898).

His wife, Princess Karoline Reuss of Greiz died on 17 January 1905, eighteen months after their marriage, under mysterious circumstances.

[5] Lottero was an Italian stage actress, and due to Hermann's rollicking lifestyle, the ducal family forced him to renounce his rights of succession to the Saxe-Weimar-Eisenach throne, as well as his royal status, title and prerogatives, granting him a lesser, noble title, Count von Ostheim, along with a small allowance on the grounds that he stay out of the duchy.

[7] Prince Wilhelm fled to the United States in his youth, served as a riding master, clerk, book agent and even as a restaurant waiter in New York City, but was finally persuaded to return to Germany, marry his second cousin, and live on a small pension from the head of the house.

Prince Wilhelm married Princess Gerta Auguste of Ysenburg and Büdingen zu Wächtersbach (1863-1945), younger daughter of Ferdinand Maximilian I, Prince of Ysenburg and Büdingen (1824-1903) and his wife, Princess Auguste Marie Gertrude of Hanau and Horowitz (1829-1887), on 11 April 1885 at Wächtersbach, Germany.