He was born at Antwerp, in a year variously reported between 1567 and 1576. He studied under Jan Snellinck, was a master in 1602, and in 1607 was dean of the master-painters. He died in the city of his birth.
Till the appearance of Rubens he was considered perhaps the best historical painter of his time. The styles of the two artists are not unalike. In correctness of drawing Janssens excelled his great contemporary; in bold composition and in treatment of the nude he equalled him; but in faculty of color and in general freedom of disposition and touch he fell far short. A master of chiaroscuro, he gratified his taste for strong contrasts of light and shade in his torchlights and similar effects. Good examples of this master are to be seen in the Antwerp museum and the Vienna gallery. The stories of his jealousy of Rubens and of his dissolute life are quite unfounded.