Leonhard Ennen's parents were simple farmers, the equivalent of sharecroppers in German agricultural society.

Following his ordination, he served as vicar and head of the upper Stadtschule, city school, in Königswinter am Rhein, and he remained in this position from 1845 to 1857.

His superiors at the education ministry sent him to Paris, to study archive development and management: the fruit of this study, Frankreich und der Niederrhein, oder Geschichte von Stadt und Kurstaat Köln seit dem 30jährigen Kriege bis zur französischen Occupation, appeared in two volumes in 1855 and 1856.

This study of France's relationship with the states of the lower Rhine during the Thirty Years War attracted attention to his talents as an historian, and on 1 August 1857 he was appointed director of the Historisches Archiv der Stadt Köln (Historical Archive of the City of Cologne).

He continued to identify, categorize, and publish volumes on the sources and documents of the history of Cologne.