Zieleniec spent most of his professional career as an
economist. From 1973 until the
fall of the communist regime in 1989, he worked first as a research fellow in the Institute of Research in Engineering Technology and Economics Research, and later at the Institute of Economics of the
Czechoslovak Academy of Sciences. Apart from research in microeconomic theory, he worked on the problems of economic transition from a
centrally planned to
market economy. The study "Czechoslovakia at the crossroads", based on his research, had a significant impact on the discussions about economic reforms after 1989. At the beginning of 1990 Zieleniec co-founded the
Center for Economic Research and Graduate Education, the first American-style economics doctoral program in Central and Eastern Europe, at
Charles University in Prague. He became its first director, and was also appointed senior lecturer in
economics and joined the Scientific Council of the
Faculty of Social Sciences at
Charles University. From 1998 to 2003 he was a businessman. ==Political career==