With the approval of Tsar Nicholas I, Niko was placed under the regency of his mother, Princess Ekaterina; the Russian bureaucrat Kornely Borozdin was assigned to him as a tutor.

[2] Niko remained in St. Petersburg, enjoying the favor of the imperial family[2] even after his mother hurried home due to a peasant revolt in Mingrelia in May 1857.

On his return, Niko, persuaded to accept a fait accompli, renounced his hereditary title of Prince Regnant of Mingrelia on 4 January 1867.

With this, Mingrelia was formally annexed directly into the Russian Empire; Dadiani retained his palaces in Zugdidi and Gordi as his personal property.

[2] In 1875, Prince Dadian-Mingrelsky was transferred to the Chevalier Guard Regiment and saw action as part of the detachment of General Iosif Gurko in Bulgaria during the 1877–1878 war with the Ottoman Empire.

After the war, he withdrew to reserves with the promotion to major-general in August 1878 and finally retired from service with a privilege of wearing a uniform in October 1878.

The Russian nominee was rejected by the Grand Assembly of Bulgaria, and the crown eventually went to the German prince Ferdinand of Saxe-Coburg and Gotha in July 1887.