Janusz Lewandowski graduated in economics from University of Gdańsk, received a doctorate and was a lecturer there until 1984. Later he worked for Polish Oceanic Lines (Polskie Linie Oceaniczne). He was also a lecturer at Harvard University and the founder of the Research Institute for Market Economy in Gdansk (Instytut Badań nad Gospodarką Rynkową w Gdańsku).
He lost his parliamentary seat in the 1993 elections and worked as an international expert from 1994–1997. After the merger of KLD and Democratic Union in 1994 he become a member of Freedom Union (UW), and in 1997 member of Sejm representing UW. Together with Donald Tusk and other liberals he seceded from UW to form Civic Platform, and was elected an MP again in 2001. Since 2003 he was an observer in the European Parliament.
In the European elections of 13 June 2004 he was a candidate of Civic Platform in constituency #1 Pomeranian Voivodship and received 79,879 votes (=20.17%, the best result in the region). On 23 July 2004 he was elected chair of the Parliamentary Committee on Budgets.
Lewandowski has expressed skepticism about the thesis of anthropogenic climate change, putting him at odds with many of his colleagues in the Barroso Commission. MEPs have asked "which steps" president Barroso will take over the comments.