Kassis studied in Germany and Lebanon, earning a master's degree from the University of Mainz, and a PhD from the American University of Beirut, both in nuclear physics. He taught physics at a number of universities in Germany, Italy, France, England, Lebanon, and Jordan before joining Birzeit University in 1980. He served as President of Birzeit University from 2004 to 2010.
Kassis was a member of the Palestinian delegation to the Madrid Peace Conference in 1991, and a deputy head of the negotiation delegation at the Washington talks in 1992 to 1993. He later became the Director-General of the Technical and Advisory Committee to the Palestinian negotiation team, established to do preparatory work for the transition to Palestinian self-government. He was a member of the Palestinian delegation on final status negotiations in 1999.
As a proponent of transparency and reform, Kassis was appointed member of the Special Committee for the Investigation of the Auditor's Report in 1997. He later entered government as a political independent, serving as minister without portfolio in 1998, Tourism Minister in 2002, Planning Minister from 2003 to 2005, and Finance Minister from 2012 to 2013.
- "Nabil Kassis". Jerusalem Media and Communication Centre. January 4, 2010. Retrieved 2014-09-13.
- "Past Presidents". Birzeit University. Retrieved 2014-09-13.
- "Palestinian Authority premier Salam Fayyad gives up finance post". Los Angeles Times. May 16, 2012. Retrieved 2014-09-13.
- "PA's finance minister quits as West Bank economy worsens". The National (Abu Dhabi). March 3, 2013. Retrieved 2014-09-13.