Mary was born in 1424 as the eldest daughter of Lord
John II of Looz-Heinsberg and his second wife Countess Anne of Solms. Her older halfbrother
John was
Prince-bishop of
Liège. for the tapestry series with the genealogy of the House of Nassau, 1528–1530.
Getty Center,
Los Angeles. Mary married on 7 February 1440 to
Count John IV of Nassau-Siegen (
Breda Castle, 1 August 1410 –
Dillenburg, 3 February 1475), the eldest son of Count
Engelbert I of Nassau-Siegen and Lady
Joanne of Polanen. In the period between the
marriage and the death of John's father in 1442, Mary and John lived in the house
De Herberghe in the Reigerstraat in
Breda. On 22 February 1447 John and his brother Henry divided their possessions, whereby John received the possessions in the Netherlands, When the count's family started staying in the county more often, a
court was established. John had
Dillenburg Castle – which until then had been used primarily as a
stronghold against the unruly local
nobility – extended in the period 1453–1467 and rebuilt into a residential castle for the count's family. In 1464 Mary had a church chair made in the castle
chapel and a
shrine for the
Blessed Sacrament. In 1469 Mary sent for
rosemary and other
herbs to be brought to Dillenburg from the
Rhine (
Cologne or
Koblenz), presumably because of the
plague. This is the oldest surviving report in the accounts about
medicines. Through the marriage to Mary, John obtained the
heerlijkheden of
Herstal,
Vught,
Gangelt,
Waldfeucht and the , so that he possessed ¼ of the
Duchy of Jülich. This led to a dispute with John's distant relative Count
John II of Nassau-Saarbrücken, who was married to Lady Jeanne of Looz-Heinsberg, the daughter and
heiress of Lord
John IV of Looz-Heinsberg. At Mary's husband's request,
Emperor Frederick III declared on 28 May 1470 that by granting the entire Duchy of Jülich to Duke
Gerhard VII, the claims of Mary and her sister Philippa of Looz-Heinsberg, were not to be affected. 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. 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. In 1474, the Emperor withdrew the order to Charles the Bold and instead transferred the matter to
Archbishop John II of
Trier. The dispute was only settled when on 25 August 1499 Mary's eldest son
Engelbert II transferred his half of the castle and the land of Millen with the towns of Gangelt and Vught to Duke William of Jülich and Berg and received in exchange from the latter on 27 August 1499 the city and the land of
Diest and the castle and the land of
Zichem and
Zeelhem. As a
widow Mary founded Vredenburg Abbey in
Bavel in 1476, She died in Siegen on 20 April 1502 GroteKerkBreda.jpg|The
Grote Kerk in Breda, 2012. Grote Kerk Breda 2017 04.jpg|The epitaph for Engelbert I of Nassau-Siegen, Joanne of Polanen, John IV of Nassau-Siegen and Mary of Looz-Heinsberg, in the
Grote Kerk in Breda. Photo: Richard Broekhuijzen, 2017. ==Issue==