He attended secondary school at the Jesuit St Jozef college in Turnhout, Belgium after which he decided to follow in his father's footsteps and become a physician.

During World War II Janssen studied physics, biology and chemistry at the Facultés universitaires Notre-Dame de la Paix (FUNDP) in Namur.

After he returned to Belgium, he worked part time at the University of Ghent Institute of Pharmacology and Therapeutics, headed by Corneille Heymans, who had won the Nobel Prize for medicine in 1938.

[citation needed] With a loan of fifty thousand Belgian francs received from his father, Janssen founded his own research laboratory in 1953.

[10] In 1995, together with Paul Lewi, he founded the Center for Molecular Design, where he and his team[11] used a supercomputer to search candidate molecules for potential AIDS treatments.