Born in Altjessnitz, Kersten joined the Social Democratic Party of Germany (SPD) and a trade union in 1946.
The SPD in East Germany was merged into the Socialist Unity Party of Germany, but Kersten remained part of a social democratic opposition group in the party.
He then moved to Frankfurt in West Germany, and found work with the Bundesbank.
He became the secretary of the department, and also the chair of the confederation's International and European Committee.
In 1972, he was elected as general secretary of the International Confederation of Free Trade Unions, serving until 1982, when he retired, due to poor health.