He was the son of Mirko Petrović-Njegoš, a celebrated Montenegrin warrior (an elder brother to Danilo I of Montenegro) and his wife, Anastasija Martinovich (1824–1895).

A change was introduced by Danilo I, who declined the episcopal office, married and declared the principality hereditary in the direct male line.

Prince Nikola, who had been trained from infancy in martial and athletic exercises, spent a portion of his early boyhood in Trieste at the household of the Kustic family, to which his aunt, the princess Darinka, wife of Danilo II, belonged.

Unlike his contemporary, King Milan of Serbia, Prince Nikola was little influenced in his tastes and habits by his Parisian education; the young highlander, whose keen patriotism, capability for leadership and poetic talents early displayed themselves, showed no inclination for the pleasures of the French capital, and eagerly looked forward to returning to his native land.

At age 19, in Cetinje, on 8 November 1860, he married Milena, 13 years old, daughter of a Vojvoda named Petar Vukotić and wife Jelena Vojvodić.

In 1867 he met the emperor Napoleon III at Paris, and in 1868 he undertook a journey to Russia, where he received an affectionate welcome from the tsar, Alexander II in St Petersburg.

[7] His efforts to enlist the sympathies of the Russian imperial family produced important results for Montenegro; considerable subsidies were granted by the tsar and tsaritsa for educational and other purposes, and supplies of arms and ammunition were sent to Cetinje.

In 1869 Prince Nikola, whose authority was now firmly established, succeeded in preventing the impetuous highlanders from aiding the Krivošijans in their revolt against the Austrian government; similarly in 1897 he checked the martial excitement caused by the outbreak of the Greco-Turkish War.

In 1876 Nikola declared war against Turkey; his military reputation was enhanced by the ensuing campaign, and still more by that of 1877/78, during which he captured Nikšić, Bar and Ulcinj.

According to Bolati, the Montenegrin court was not grieving that much over the murder of King Alexander Obrenović, as they saw him as an enemy of Montenegro and obstacle to the unification of Serb Lands.

A few days later, Serbia (including Montenegro) merged with the former South Slav territories of Austria-Hungary to form the Kingdom of Serbs, Croats and Slovenes, which was renamed Yugoslavia in 1929.