From 1962, Janssens composed religious songs of the genre later called Neues Geistliches Lied, beat masses and later musicals. He wrote numerous songs, children's songs such as "Herr Uklatsch", and musicals such as Dietrich Bonhoeffer – ein Leben im Widerstand (Dietrich Bonhoeffer – A Life in Resistance). He set music to texts by authors such as , , Ernesto Cardenal, , and . He introduced the term "" to Germany, when in 1972 he subtitled his Menschensohn (Son of Man) Ein Sacro-Pop-Musical (A sacropop musical). From 1973, Janssens performed with his musical ensemble Gesangsorchester at the Kirchentag, first in Düsseldorf. In 1992, he wrote Passion der Eingeborenen (Passion of the Indigenous), related to 500 years after the European invasion in Latin America. He was a member of the association which is now the Textautoren- und Komponistengruppe (TAKT).
