Kostomarov was also known as one of the main figures of the Ukrainian national revival society best known as the Brotherhood of Saints Cyril and Methodius,[7][2][8][9][10][11] which existed in Kiev from January 1846 to March 1847.

Kostomarov was also a poet, ethnographer, pan-Slavist, and promoter of the so-called Narodnik movement in the Russian Empire.

In his various historical writings, Kostomarov was always very positive about Kievan Rus' and Novgorod Republic, about what he considered to be the veche system of popular assemblies (see especially his monography On the role of Novgorod the Great in the Russian history (Russian: О значении Великого Новгорода в русской истории), and the later Zaporozhian Cossack brotherhood, which he thought in part was an heir to the democratic system as well.

His detailed writing on the case of Stepan Razin, one of the most popular figures in the history of the Don Cossack Host, was particularly important for the political evolution of Narodniks.

Kostomarov maintained a long-standing argument with Mikhail Pogodin regarding the linguistic and ethnographic origin of the word "Rus'".

The argument between Kostomarov and Pogodin about the origin of Rus' had an influence on the building of two different historiographical schools in Russia: the so-called ″Normanists″ and ″Anti-Normanists″.

In the 1840s, he helped to found an illegal political organization called the Brotherhood of Saints Cyril and Methodius in Kiev (for which he suffered arrest, imprisonment, and the exile to Saratov).

[16] From 1847 to 1854 Kostomarov, whose interest in the history of Little Russia and its literature made him suspected of separatist views, wrote nothing, having been banished to Saratov, and forbidden to teach or publish.

In 1862, he was forced to resign from his post as chair of department of history of the University of Saint Petersburg,[17] because he had sympathized with the revolutionary movement of liberals, progressives, and socialists.

He published two poetry collections under the pseudonym Yeremiia Halka, Ukrainski baliady (Ukrainian Ballads, 1839) and Vitka (The Branch, 1840), both containing historical poems mostly about Kievan Rus' and Bohdan Khmelnytsky.