He was ordained priest in 1860; became a member of the Prussian House of Deputies in 1875; and in 1880 was made domestic prelate to the pope and apostolic pronotary. He died at Frankfurt. Janssen was a stout champion of the Ultramontane party in the Roman Catholic Church.
His great work is his Geschichte des deutschen Volkes seit dem Ausgang des Mittelalters (8 vols., Freiburg, 1878-1894). In this book he shows himself very hostile to the Reformation, and attempts to prove that the Protestants were responsible for the general unrest in Germany during the 16th and 17th centuries. The author's partisanship led to some controversy, and Janssen wrote An meine Kritiker (Freiburg, 1882) and Ein zweites Wort an meine Kritiker (Freiburg, 1883) in reply to the Janssens Geschichte des deutschen Volkes (Munich, 1883) of M. Lenz, and other criticisms.
The Geschichte, which has passed through numerous editions, has been continued and improved by Ludwig Pastor, and the greater part of it has been translated into English by M. A. Mitchell and A. M. Christie (London, 1896, fol.). Of his other works perhaps the most important are: the editing of Frankfurts Reichskorrespondenz, 1376-1519 (Freiburg, 1863-1872); and of the Leben, Briefe und kleinere Schriften of his friend JF Böhmer (Leipzig, 1868); a monograph, Schiller als Historiker (Freiburg, 1863); and Zeit- und Lebensbilder (Freiburg, 1875).