On 20 January 1827, Prince-Bishop Petar I nominated Radivoje as his successor instead of Đorđije Savov, who went to the Russian Empire and became a cavalry officer. Petar wanted to dispatch Njegoš to Russia, but he lacked the much-needed finances, so he decided to educate Rade himself. He taught him Italian, Russian and German, as well as the basics of English and French. Petar also gave Rade access to his rich library. The Prince-Bishop assigned one of the greatest Serb writers of the time, Sima Milutinović Sarajlija (of Sarajevo, Bosnia and Herzegovina, then part of the Ottoman Empire) to be Radivoje's new teacher. He was taught the Classics, art, history, philosophy and literature.
The next day, on 20 October, Radivoje buried his uncle. The same day, Rade became a monk under the Archimandrite of the Monastery of Vranjina and took the cloak of his deceased uncle. Two days later, Rade himself became an Archimandrite, becoming the unofficial supreme ecclesiastical ruler of Montenegro. On 30 October the same year, he sent a letter to Jeremija Gagić declaring his assertion to power:
Prince-Bishop Radivoje took over the leadership over the Serb clans of four districts: Katunska Nahija, Lješanska Nahija, Riječka Nahija, and Crmnica, as well as four Highland tribes: Bjelopavlići, Piperi, Rovčani, and Moračani. He was only the ecclesiastical ruler over Boka and the Skadar. He was still young, so his father Tomo and his uncle Captain Lazar Proroković assisted him as well as some major Chiefs.
At the end of 1830 and beginning of 1831, Venetian-imposed Governor Vukolaj Radonjić moved against Radivoje wishing to end the House of Njegoš's dominance over Montenegro. On 22 November 1830, Prince-Bishop Rade wrote a letter to Jeremija Gagić regarding the Governor's rebellion:
On 31 January 1831 on the island of Kom in the Monastery of Vranjina, the Archbishop of Rascia-Prizren declared him as the official Archimandrite. Radivoj received the name Petar II in his predecessor's honour. Prince-Bishop Petar II invited two Serbian envoys in the Russian Empire to come and assist him in his reign: Mateja Vučićević, Montenegro's viceroy in Russia and his uncle, Ivan Vukotić, a subofficer in the Russian Army.
The two arrived to Montenegro in September the same year and on 27 September brought the decision on assembling the governmental infrastructure in Montenegro. A Senate was formed, headed by the Prince-Bishop and composed out of 16 Senators - the most prominent Montenegrin Chiefs. The Senate's duty was to act as a Government and the Supreme Court. A Guard was formed that acted as the Executive branch of the government that had 164 members that served as the Police and travelling judges in minor conflicts. On 6 December 1831, Peter II wrote to Jeremija Gagić regarding these reforms (in Serbian):
Ivan Vukotić became the first President of the Senate, while Mateja Vučevićević became its first Vice-president. The Senate's seat was in Cetinje, while the Guard's Headquarters were in Rijeka Crnojevića. Petar II was present on every assembly of the Senate except judgments of capital sentences, in which he was forbidden to participate by the canon law. Prince-Bishop Petar II later formed Captains to monitor the Serb clans in his domain as well as the Grenadiers(Perjanici) - the Prince-Bishop's personal elite guard. He also formed a special Border Militia (Serbian: Panduri or Пандури) to patrol the borders of Montenegro.
Up to 1832, Petar fully cancelled governorship, therefore affirming full power over Montenegro. Petar II renamed the Praviteljstvo suda into Praviteljstvujušći sovjet, expanding its powers from just courtial to also management. He also founded captains for every Serb clan in Montenegrin - his own representatives in these Serbian clans.
Prince-Bishop Petar II wanted to raise Montenegro's international prestige. In order to achieve that, he visited the Russian Czardom in 1833, after a brief stop in Vienna, where he was accepted into Ecclesiastical service as Prince-Bishop of Montenegro in Saint Petersburg, the Empire's capital. In 1833, just before his journey to Serbia, Serbian Orthodox Christian Bishop of Užice gifted him Danica of Vuk Karadžić from 1826.
Peter II has contributed greatly to education by founding the Elementary School in Cetinje, Montenegro's capital in 1834. The same year, 1834 - he opened a Printing Press in Cetinje specifically for printing his works - the same year printing the Hermit of Cetinje. In 1835, the Montenegrin forces captured a cannon in Žabljak and Peter II ordered a little poem to be engraved in it (in Serbian):
In 1836, he paid another visit to the Russian Empire, again making a short stop in Vienna. The same year, 1836, he published The ABC of the Serbian language. In 1838 he also published The Serbian Grammar. He also re-printed the school textbooks originally printed by his uncle Petar I Njegoš The Serbian elementary reading book.
The conflicts with the neighbouring Muslims of the Ottoman Empire were insignificant - except the epic struggle with Smail-aga Čengić in 1840 on Mljetičko. Peter could not achieve the high statehood of his predecessor - the Serbian Orthodox Christian monasteries of Maina and Stanjevići were bought by the Empire of Austria, while Vranjina and Lesandro were seized by the Pasha of Skadar. Although Peter II always supported rebels against the Ottoman authority and gladly went to openly fight the Ottomans, Russia's peaceful policy towards the Ottoman Empire meant that no larger martial success could be undertaken.
In 1842, Prince-Bishop Peter II constructed another elementary school - in Dobrsko Selo. On the 11 June 1842, the Prince of Serbia Mihailo Obrenović and the Serbian Literature Society elected him as an "Honorable Member" as a reward for his merits in literature and education of the Serbian people. Later, in 1845, he was declared the Metropolitan of Cetinje. The same year, 1845, Peter II published the Light of Microcosm, an impressive, masterfully written philosophical work. In 1846, Peter wrote a collection of Montenegrin national poems - the Serbian mirror in honor of one of the greatest Russian writers - Pushkin.
In 1846 and 1847, Peter II was in Vienna, the Austrian Imperial capital. There, he published in 1847 The Mountain Wreath - his most famous work. It described the will of the Serbian people to fight for freedom in 2,819 verses. The same year, 1847, Njegoš wrote the Of false Tsar Stephen the Little, where he described the life of the first uniter and ruler of modern Montenegro - Czar Stephen the Little from the 18th century.
In 1848, the government of the Principality of Serbia sent him the proposal of unification of Serbs, Croats and Bulgarians. Petar agreed but said:
In late 1848 and early 1849, Prince-Bishop Peter II assisted the Revolutionary fights of Croatian Ban Josip Jelačić and maintained close ties with the Principality of Serbia. Although Peter II's outer policy completely relied on Russia, Russia maintained good relations with the Ottoman Empire - so nothing more than a reconciliation with the Ottomans could be achieved.
In 1851, Peter II caught tuberculosis. He paid a visit to the Italy the same year, 1851, attempting to find a cure. The same year, 1851, he managed to publish his last major work - Of false Tsar Stephen the Little. Peter II Njegoš died in Cetinje of tuberculosis on 31 October (October 19 Old Style) 1851 - exactly 21 year after his accession to the throne; he was buried in a small chapel on top of Mount Lovćen where his mausoleum was built. During the period of Communist rule in Yugoslavia it was demolished for ideological reasons by the authorities to make way for a secular monument.