Mauritia Eleonora was the fifth daughter and ninth of ten children of Manuel, Hereditary Prince and Countess Emilia of Nassau.

[1] Her father was the illegitimate son of António, Prior of Crato, pretender to the Portuguese throne during the succession crisis of 1580.

[5][6][7][8] She was named after her maternal uncle, Maurice, Prince of Orange, to celebrate the reconciliation between him and Mauritia Eleonora's mother.

She shared a room with Countess Louise Christine of Solms-Braunfels, the youngest sister of Frederick Henry's wife Amalia.

In November 1644, Mauritia Eleonora confirmed William Frederick's suspicion that Louise Henriette was secretly corresponding with Henri Charles de la Trémoïlle[note 2] Prince of Talmont.

[11] William Frederick did not realise that Mauritia Eleonora's had ulterior motives for giving him biased information, and he was charmed by her.

Fearing that she would somehow marry the Prince of Talmont, Amalia ordered Mauritia Eleonora to watch Louise Henriette closely.

She did her job thoroughly, and Talmont complained that he could never be alone with Louise Henriette[14] He explained Mauritia Eleonora's obedience to her aunt with her dependency: as an orphan in her mid-thirties with no assets, she needed Amalia's help to marry.

[15] In September 1646, Mauritia Eleonora told Amalia about the secret correspondence between Louise Henriette and the Prince of Talmont and received her permission or order to obtain the letters.