From 1973 to 1975 he was a graduate student in the Department of Economics of the University of Copenhagen, and in 1975 he became an assistant professor and then from 1978 to 1982 an associate professor there.
From 1982 to 1993, he was an economist with the Arbejderbevægelsens Erhvervsråd [da] (Economic Council of the Labour Movement). In 1993 he became head of the department, in 1994 the director, and later in 1994 the treasurer.
Koch was the director of Danske Bank from 1 September 2000 until 2008, when he became chairman of the Skattekommissionen [da] (Danish Tax Commission).
In 2009, Carsten Koch was appointed CEO of LD (now LD Pensions [da]) on a two-year contract.
In 2010, he was appointed president of the Beskæftigelsesrådet [da] (Employment Council).
In 2014, he was chairman of the Expert Committee on Active Employment Efforts (the Carsten Koch Committee).