In 1818, he became chief director of the educational circle of Győr (German: Raab), and in 1824 was appointed librarian to the University of Pest.

Fejér's works, which are nearly all written either in Latin or Hungarian, exceed one hundred and eighty.

[1] His most important work, Codex diplomaticus Hungariae ecclesiasticus no civilis, was published in several volumes from 1829 to 1844.

It consists of old documents and charters from 104 to the end of 1439, and forms an extraordinary monument of patient industry.

[1] His later works were A Kunok eredete (The Origin of the Cumans), and A politikai forradalmak okai (The Causes of Political Revolutions), both published in 1850.