Johann Karl was born on 19 December 1839 at Ladendorf Castle, Mistelbach District, Lower Austria.

Among his siblings were Countess Maria von Khevenhüller-Metsch (wife of Count Rudolf Chotek von Chotkowa und Wognin), Count Sigmund Maria von Khevenhüller-Metsch (who married Countess Maria-Anna von Herberstein), Countess Leontine Antonie Marie von Khevenhüller-Metsch (wife of Maximilian Egon I, Prince of Fürstenberg and, after his death, Prince Emil Egon of Fürstenberg, both sons of Charles Egon II, Prince of Fürstenberg), and Count Rudolf Khevenhüller-Metsch (who married Princess Alexandrine Windisch-Graetz, a daughter of Hugo, 2nd Prince of Windisch-Graetz).

[1] His paternal grandparents were Franz, 4th Prince of Khevenhüller-Metsch, and, his third wife (and niece), Countess Krisztina Zichy de Zich et Vásonkeő.

Upon the death of his father on 29 November 1877, he became the 6th Prince of Khevenhüller-Metsch and became a member of the Austrian House of Lords.

[8] His widow, Eduardine, died on 2 August 1925 at Opatija, a town in Primorje-Gorski Kotar County, Croatia.