Prior to 1826 he was an official at the Ministry of Internal Affairs. Later he was appointed to the Office of His Imperial Majesty. He began painting on his own. In 1809 he attended drawing classes at the Academy of Arts. In 1812 he made copies of paintings in the Hermitage. It was most probably, at this time that he became acquainted with A.G. Venetsianov who was working in the Hermitage at the same time. In 1813 he completed a series of cheap popular prints which were of a patriotic nature about the 1812 War. In 1817 he was involved in the publishing of "The Magic Lamp or a View of St. Petersburg Traders", for which he produced 40 etchings containing scenes from everyday life and popular characters. His work during the 1820s - early 1830s demonstrates A.G. Venetsianov’s influence on Zelentsov, and it is likely that he studied under him (Venetsianov). As a result of his father’s bankruptcy, in the 1830s he took up professional painting and it became his main source of income. In 1833 for his painting "Studio of Artist P.V. Basin" Zelentsov was awarded with the title of Academician. In the 1830s he worked effectively with numerous literary publishers, illustrating work for the collection of stories, "100 Russian Novelists" by A.S. Pushkin, V.A. Zhukovsky, M.N. Zagoskin and F.V. Bulgarin (1833-1841). He was the author of the book, "The Happy ABC" written and illustrated by K.A. Zelentsov". K.A. Zelentsov produced several interior paintings, which are considered to be the best examples of this genre in Russian art. Interiors were popular subjects in early 19th century art, particularly within Venetsianov's student circle. Venetsianov believed interiors a part of the nature school: "I have never urged my pupils to exclusively study perspective; the room interiors they have painted […] should have been their initial contact with nature". In his later work, Zelentsov, rigidly adhered to the principles of the Venetsianov's school, he had a strong interest in the idealised academic version of genre art.