[4] On leaving school Koenen had embarked on an apprenticeship as a Machinist-Fitter and in 1906 joined the German Metal Workers' Union.

In 1917 he joined the break-away Independent Social Democratic Party (USPD)[1] which had split primarily on account of the mainstream SPD's continuing support for the war.

During the November Revolution, Koenen was deputy leader of the Workers' Council at the Leuna Chemicals Plant where he was then working.

On 12 February 1933, during the course of the Eisleben Eisleber Blutsonntag [de] (Bloody Sunday) Nazi propaganda march, several Communist leaders were killed; Koenen, however, was merely assaulted and badly injured by SA members, as a result of which he lost an eye.

In May 1933, along with several other fugitive communists, he managed to get to the Saarland,[1] the only part of Germany still under foreign military occupation following the end of the First World War.

[1] In April 1946 he became a founder member of the Socialist Unity Party of Germany (SED / Sozialistische Einheitspartei Deutschlands).

[3] Under the Soviet system, on which the East German constitution was modelled, the responsibilities of government ministers were restricted to implementing the decisions of the Party Central Committee, while a Single List voting system ensured that the National Legislative Assembly (Volkskammer) was also controlled by the ruling SED (that is to say, by its Central Committee).

In terms of power and influence Koenen's Central Committee membership was therefore of greater significance than membership of regional or national legislatures, though in practice there were plenty of Central Committee members who simultaneously held office in national or regional legislatures and/or as government ministers.

Between 1952 and 1953, and again, in succession to Franz Bruk, from 1958 till 1963 Koenen served as First Secretary to the SED Regional Leadership (Bezirksleitung) in Halle.

[2] From 1953 to 1958 Koenen was appointed as his country's ambassador to neighbouring Czechoslovakia, an important post, succeeding Fritz Große.