Born in Wittgenstein, she was the sixth and youngest child of Ernest, Count of Sayn-Wittgenstein-Sayn and his wife, Countess Louise Juliane of Erbach.
A violent dispute over his inheritance erupted between the Dowager Countess and the three half-brothers of her late husband (sons Ksibdidi After two months, one of them, Count Louis Albert of Sayn-Wittgenstein-Neumagen, finally forced Louise Juliane to surrender the county to him and his brothers.
One year later (1652), Louise Juliane finally handed over the County of Sayn to her daughters, who was divided in two parts: Sayn-Wittgenstein-Sayn-Altenkirchen (for Johannetta) and Sayn-Wittgenstein-Sayn-Hachenburg (for Ernestine, who recently married Count Salentin Ernest of Mandersheid-Blankenheim).
After the death of Duke William (1662), his two older sons inherited almost all the family domains; as result, John George only received an income from the new duchy of Saxe-Eisenach and the small town of Marksuhl as residence.
Then, the county was inherited by Charles William Frederick, Margrave of Brandenburg-Ansbach, grandson of Eleonore Erdmuthe, Johannetta's eldest daughter.