Janssen was a descendant of the well-known banking family and was also a banker at Generale Bank all his life. In 1988 he became CEO of Generale Bank and chairman in 1992.
After Generale Bank was taken over by Fortis in 1998, he resigned.
During his career, he was chairman of the Belgian Banking Association and director and member of the management committee of the Federation of Belgian Enterprises (VBO). He served on the boards of directors of Solvay, Atlas Copco Sweden, Antwerp Diamant Bank and the Lhoist group, among others.