Johann Hahlen was born in Trier on 13 December 1942. After achieving his university entrance qualification, Johann Hahlen studied law in Freiburg from 1961 to 1965. This was followed by basic military service from 1966 to 1967, legal preparatory service from 1967 to 1970, and the second state examination in law in Stuttgart in June 1970.
From 1970 to 1972, Johann Hahlen worked as an employed lawyer in Cologne. In April 1972, he joined the Federal Ministry of the Interior, where he worked in various functions and departments: Assistant Head of Section in the following departments: Public service, constitution, constitutional law and administration, external probationary period at the Federal Office of Administration, Department of sports and media, Section for the borders commission, transit commission, and Berlin affairs, Department for police affairs. From 1983 to 1988 he worked for the working group on domestic affairs of the CDU/CSU Bundestag parliamentary group. During that time, he was on leave from the Federal Ministry of the Interior. From 1988 to 1992, he was Head of the budget Section of the Federal Ministry of the Interior, and from 1992 to 1995 Head of the Sub-Directorate for constitutional law at the Federal Ministry of the Interior.
From 2 October 1995 to 5 October 2006, Johann Hahlen was President of the Federal Statistical Office and until 18 January 2007 Federal Returning Officer. Under Johann Hahlen, the statistics agency developed into a modern information provider. Methodical reorientation and the use of technology are intended to reduce the burden on respondents. Also, modified management tools and more importance attached to the communication of statistical results are changing the image of official statistics. The change from a data producer to a data provider is illustrated by the introduction of the new corporate design of the Federal Statistical Office in 2001 with the logo “DESTATIS – wissen.nutzen” (DESTATIS – use.knowledge).
From October 2006 to December 2007, Johann Hahlen was State Secretary at the Federal Ministry of the Interior.