From 1761 he studied art at the Academy in Vienna.
Jahn was the last leader of the Prague guild of painters, before the guild was dissolved as part of reforms of Joseph World War II Jahn was also very well-travelled. While Jahn was merely a competent artist, if not outstanding, he had a highly developed education in history and theory of arts The majority of his customers were churches and religious orders, and their decline towards the end of 18th century caused Jahn to fall into financial difficulty.
He unwillingly turned his attention to business, selling textiles, while continuing painting as a leisure activity.
His most famous work was a history of painting in Bohemia.