Janssen studied econometrics and philosophy of economics at the University of Groningen, where he received his PhD in 1990.
From 1997 to 2008, he was professor of microeconomics at Erasmus University Rotterdam and director of the Tinbergen Institute from 2004 to 2009.
[3][4] In addition, Janssen has launched a new subfield that studies the effects of consumer search in vertically structured industries.
[9] Janssen is a member of the Dutch scientific Royal Holland Society of Sciences and Humanities.
[10] In 2017, the National Research University Higher School of Economics in Moscow awarded Janssen an honorary degree.