Mary was born in 1424[1][2][3][4][note 2] as the eldest daughter of Lord John II of Looz-Heinsberg and his second wife Countess Anne of Solms.

[25] Through the marriage to Mary, John obtained the heerlijkheden of Herstal, Vught, Gangelt, Waldfeucht and the Land of Millen [de], so that he possessed ¼ of the Duchy of Jülich.

[27] Apparently this had no effect, because in 1471 or 1472 the Emperor ordered Duke Charles the Bold of Burgundy to settle on his behalf the dispute between Gerhard on the one hand and Mary and John on the other.

[28] The dispute became more complicated when in 1472 Gerhard VII's son William married to Countess Elisabeth of Nassau-Saarbrücken, John II's eldest daughter, who had inherited her mother's possessions.

[26][29][30] In 1474, the Emperor withdrew the order to Charles the Bold and instead transferred the matter to Archbishop John II of Trier.

[30][32] As a widow Mary founded Vredenburg Abbey in Bavel in 1476,[2][5][6][7][14] and the convent of the Grey Sisters [nl] in 1478.