Mary was born on 2 February 1418[1][2][3][4] as the second daughter and fifth child of Count Engelbert I of Nassau-Siegen and his wife Lady Joanne of Polanen.

[1][3][4] Mary got engaged on 6 January 1428[4][5] and married in Breda[1][2][3][4] on 17 June 1437[1][2][3][4][5] to Count John of Nassau-Wiesbaden-Idstein[note 2] (1419[1][2][3] – 9 May 1480[1][2][3][4][5]).

On 14 October 1463, the Emperor appointed Abbot Reinhard [de] of Fulda as his attorney and instructed him to summon both parties to a trial on his behalf, to interrogate them and to give judgment.

What is known is that the Emperor summoned John for the settlement of the inheritance dispute with his sister, because on 9 February 1467 Mary appointed attorneys for the trial.

At the edge of the epitaph was the inscription:'+ Anno d(omi)ni m cccc Ixxii / die ii me(n)s(i)s octobris o(biit) G(e)n(er)osa d(omi)na maria ac nassawe comitissa i(n) nassawe / c(uius) a(n)i(m)a req(ui)escat i(n) pace.