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.