Janssen was born in Goch, Germany, near the Dutch border. He was ordained a priest in 1861. Janssen purchased land in Steyl (Netherlands) to begin his seminary, dedicated in 1875 as "St. Michael the Archangel Mission House". Within a few years, many seminarians, priests and brothers were preparing for missionary service there, and the first two missionaries, Joseph Freinademetz and John Anzer, were sent to China .
St. Janssen also founded two congregations of religious Sisters: The Holy Spirit Missionary Sisters (members known as "Sister Servants of the Holy Spirit") in December, 1889, and the Holy Spirit of Perpetual Adoration ("Sister Servants of the Holy Spirit of Perpetual Adoration") in 1896.