Following the annexation of the Duchy of Nassau by the Kingdom of Prussia as a consequence of the Austro-Prussian War in 1866, he entered into Prussian military service the following year.

In 1868, he entered into a morganatic marriage with the youngest child of the Russian poet Alexander Pushkin, Natalia Alexandrovna Pushkina, who was created Countess of Merenberg.

The subsequent claims of inheritance and succession in the Grand Duchy of Luxembourg of the children born of this marriage later developed into the socalled Merenberg Affair.

His granddaughter, Countess Nadejda Mikhailovna de Torby married George Mountbatten, 2nd Marquess of Milford Haven in 1916.

[1] Following the expulsion of Otto of Greece in October 1862, the government of the United Kingdom briefly considered making Prince Nikolaus its candidate for the vacant Greek throne.

[3] Prince Nikolaus was promoted to colonel on 1 January 1864 and to general major à la suite on 1 February 1865.

[1] In 1856, Prince Nikolaus Wilhelm represented the Duchy of Nassau at the coronation of Emperor Alexander II in Russia.

In December 1867, Prince Nikolaus informed his half-brother, Duke Adolph, about his marriage to Natalia which was to have taken place “some time ago”.

[1] In 1890, Prince Nikolaus' half brother, Duke Adolf inherited the throne of the Grand Duchy of Luxembourg upon the male-line extinction of the Dutch line of the House of Nassau.

And in 1895, Prince Nikolaus' son, Count Georg of Merenberg, married Princess Olga Alexandrovna Yurievskaya (1874-1925), the morganatic daughter of Emperor Alexander II of Russia.

The only heir in the male line of the Grand Duke was his cousin, Prince Nikolaus' son Count Georg of Merenberg.

But the young man was born from a morganatic marriage, and William IV preferred to declare him a non-dynast and repeal the Salic law.