Contrary to Henry's wishes, the Sayn family made claims to the Sayn estate shortly after his death and by 29 August 1247, Mechthild left to the sons of her sister-in-law, Adelheid and the others, the castle and town of Blankenberg, the castle and lands of Hachenburg, Freusburg Castle, Sayn Castle, the castles of Saffenburg and Hülchrath and all the counties and bailiwicks that Henry had owned. Mechthild retained her own Thuringian inheritance and reserved the right to live at the Sayn castle of Löwenburg in the Siebengebirge. The castle of Waldenburg and the villages of Drolshagen and Meinerzhagen were sold on 20 January 1248 to the Archbishop of Cologne, Conrad of Hochstaden for 2,000 Cologne marks. Mechthild initially held the castles of Altenwied, Neuerburg, Rennenberg and Windeck as well as the villages of Rosbach, Linz, Leubsdorf, Neustadt, Asbach, Winden, Windhagen, Gielsdorf, Sechtem, Nieder- and Oberbreitbach as well as scattered lands and vineyards along the rivers Rhine and Moselle. On 1 May 1250, Mechthild signed a contract with the Elector of Cologne, Conrad of Hochstaden, at the Neuerburg, according to which all the estates around the castles of Altenwied, Neuerburg, Rennenberg and Windeck passed to the Archbishopric of Cologne after her death in return for a one-off payment of 600 marks and an annual payment of 170 marks. The Neuerburg and the church parish of Breitbach were available for her lifelong use. On 2 March 1261, Conrad's successor, Archbishop Engelbert I renewed the treaty, which was confirmed by Pope Urban IV in 1263. Mechthild lived at the Neuerburg for a few more years and later moved to Cologne, where she had a residence at Sion Abbey. In 1283, in her will, she decreed that after her death her estate would eventually pass to the Archbishopric. Her date of death is unknown, according to an unconfirmed interpretation of a gravestone in Cologne she may have lived until 1291. The most important foundations and gifts that Mechthild and her husband made were:  Mechthild's deeds were often recorded in the German language which was exceptional in the 13th century.