Lönnrot spent the most energetic years of his adult life in Kainuu. He was 32 when he moved to Kajaani and 51 when he returned to Helsinki to pursue a career at the university. The bulk of his writing also took place during this period. Naturally, his crowning achievements are the Kalevala (The Old Kalevala (1835) and the new Kalevala (1849)) and the Kanteletar (1840), but he accomplished a great deal more in the literary sphere. He was also one of the foremost journalists of his time.

While working as district physician in Kajaani, Lönnrot contributed a total of 98 articles to eleven newspapers and magazines throughout Finland these articles often being spread out over several issues. He contributed most diligently - a total of 30 articles - to Helsingfors Morgonblad, which was edited by J.L. Runeberg. Other newspapers he submitted articles to between 1833 and 1855 included Wiikko-Sanomat in Oulu, Sanan Saattaja in Vyborg, Borgå Tidning, Suomi, Saima, Maanmiehen Ystävä, Finland Allmänä Tidning, Suometar, Litteraturbladet and Kanava. In addition, he founded his own magazine, Mehiläinen, publishing a total of 42 issues in 1836-37 and 1839-40. He was also temporarily an editor for Wiikko-Sanomat in Oulu (1852-1853) and Litteraturbladet (1847-49). He published his histories of the world, Finland and Russia as supplements to Mehiläinen. He also presented Kainuu and its history to his readers. He hired assistants for his magazine from Kainuu.

In addition to the Kalevala and the Kanteletar, Lönnrot published two extensive works on folklore while he lived in Kainuu: The Proverbs of the Finnish People (1842) and The Riddles of the Finnish People (1844). Other works dealing with folk poetry were his Om närvarande tids poesie hos Finska Allmogen (Modern Poetry ...), which appeared in 1842, and Paavo Korhonen's Wiisikymmentaä runoa ja kuusi laulua (Fifty Poems and Six Songs (1848))

One literary project which Lönnrot worked on for many years was a Swedish-Finnish-German dictionary (Svensk, Finsk och Tysk Tolk, 1847), which was of great importance to the educated classes in Finland as they gradually changed languages from Swedish to Finnish. The dictionary was also the basis for another extensive work, the Finnish-Swedish dictionary, which came out in 1874 and remained a basic reference work through the beginning of this century. While working in Kajaani, Lönnrot edited another dictionary, a Russian-Swedish-Finnish pocket dictionary, which was published in 1851.

Lönnrot coined many new words in Finnish:, the word kirjallisuus (literature) is his creation, as are pääte (grammatical ending), luku (chapter, number), yksikkö (grammatical singular) and monikko (grammatical plural). He also coined a large number of words in the field of medicine and made them established usage: potilas (patient), oire (symptom), kuume (fever), valtimo (artery) and laskimo (vein).

Lönnrot wrote many articles and other publications in the fields of medicine, health care and public education, and a good number of these enjoyed long currency as practical guidebooks among the people. The first of such works was his translation Gustava Schartan'n Hyväntahtoisia Neuwoja Katowuosina (Gustav Schartan's Good Advice for Lean Years) inspired by the period of amine he encountered when he arrived in Kainuu. This was followed by Suomalaisten Talonpojan Koti-Lääkäri (The Finnish Peasant's Home Doctor) (1839) and another translation Neuvoja yhteiselle Kansalle Pohjanmaalla pienten lasten kasvattamisesta ja ruokkimisesta (Advice to the People in Ostrobothnia on Rearing and Feeding Children) (1844)

Lönnrot's scientific publications at the time he was in Kainuu include his doctoral dissertation Om Finnarnas Magiska Midicin (On the Magic Medicine of the Finns) published in a revised and expanded version in the Finnish Medical Society's series in 1842 and Om det Nord - Tschudiska språket (1853) (On the Northern Chudish Language), hastily completed in fulfillment of the requirements for a professorial chair, and the last work he would write while in Kajaani.

Lönnrot was also a pioneer in creating different genres of literature. The tale of Vorna which he published is one of the first short stories in Finnish literature.

One of the assistants for Mehiläinen magazine in Kajaani was a priest in Kianta, Carl Saksa, who was also an accomplished translator. At Lönnrot's request, Saksa wrote an extensive description of the district he was responsible for, i.e., Kianta (Suomussalmi), and Lönnrot published this account in the April 1837 issue of the magazine.