He was born at Düsseldorf, son of the engraver Theodor Janssen (1817–1894), by whom he was first instructed before studying at the Academy under Karl Sohn (Ger. wiki) and Bendemann. He is principally known through a series of decorative works whose monumental style and sound naturalism won him a reputation as one of the foremost historical painters of his time. He became professor at the Düsseldorf Academy in 1877 and its director in 1895, and was elected a member of the Prussian Academy of Arts in 1885. In Berlin he worked also for Emil Hünten. His more important mural paintings include:
- "The Colonization of the Baltic Coast" (1872), in the exchange at Bremen
- "The Myth of Promethus," in 12 compositions, in the second Cornelius Room of the National Gallery in Berlin
- "Seven Momentous Episodes in the History of Erfurt" (1882), Town Hall, Erfurt
- "Human Life," "Imagination," "Beauty," and "Nature," in the Aula of the Düsseldorf Academy
Of his oil paintings, the "Denial of Peter" is in the Academy at Philadelphia; the "Infancy of Bacchus" (1882) excited great admiration at the International Exhibition in Munich; and "Walther Dodde and the Peasants of Berg before the Battle of Worringen, 1288" (Düsseldorf Gallery), a composition of great dramatic power, containing many life-size figures, was awarded the great gold medal in Berlin in 1893.
He was the brother of sculptor Karl Janssen whose works include the monument to the Kaiser at Düsseldorf, and "Woman Hewing Stone," in the National Gallery, Berlin.
Walter Dodde and the Bergish Peasants at the Battle of Worringen, 1288 (Düsseldorf, 1893)