Koren was a working student at the Institute for Economic Research from mid-August 1944 and was employed there on June 1, 1945, where he worked until the mid-1970s. He was initially assigned to the Energy Economics and Industry departments, before devoting himself to empirical economic research and political-theoretical analyses. In 1964, he habilitated in economics at the Faculty of Law and Political Science at the University of Vienna with a thesis on nationalization in Austria. In spring 1965, he was appointed full professor of economic and social policy at the Faculty of Law and Political Science at the University of Innsbruck. Koren commuted back and forth between the two universities, and in 1968 he joined the University of World Trade (now Vienna University of Economics and Business), but was removed from this position due to his political activities.

With his hard currency policy, he shaped the Austrian economic policy landscape and in 1967 drew up an economic concept for the ÖVP, the so-called "Korenplan". One year later, Koren became State Secretary in the Federal Chancellery, and from 1968 to 1970 also Finance Minister in the government of Josef Klaus. From 1970 to 1978, he was a member of the National Council during Bruno Kreisky's government, the chairman of the ÖVP parliamentary group and thus leader of the opposition. In 1978, Koren finally took over the office of President of the Austrian National Bank, which he held until his death.