Sierau worked at the State Ministry of Urban Development, Housing and Transport of North Rhine-Westphalia in 1986. He was promoted to Ministerial in 1993 and in October 1993, at the request of the SPD parliamentary group on leave to participate in the "Government Programme 1994". From 1994 to 1999, he was director of the Institute for Regional and Urban Development Research of North Rhine-Westphalia (ILS) in Dortmund. From 1 May 1999 to 31 March 2005, he was part of Environment and Planning Department of the City of Dortmund. On 15 February 2007 he was appointed City Manager of the City of Dortmund.
Mayor of Dortmund, 2008–present Within the SPD Dortmund developed in 2008 a discussion of possible candidates for the post of Mayor in the forthcoming local elections. In an intra-party members survey, Sierau sat with 64% through to the second candidate Jörg Stüdemann and was officially elected on the SPD subdistrict Party with 97.4% of the vote for the SPD mayor candidate. On 30 August 2009, Sierau had 45.5% of the vote, succeeding (SPD) as mayor of Dortmund. After his election, Sierau was criticized because he was accused of being aware of significant funding gaps in the budget of the city of Dortmund prior to his election and keeping this knowledge private before the election. Despite the considerable criticism of this behavior, he assumed the post as mayor. After a member appointed by the city of Dortmund Council expressed their opinion on the election affair and had recommended a repetition of the local elections in Dortmund, Sierau agreed to a new election. This was decided by the city of Dortmund Council on 10 December 2009. On 19 January 2010, Sierau resigned from his duties. A person from the social department, Siegfried Pogadl, headed in his function as a city manager to manage the city. Sierau joined his former position as Planning Department of the city again. On 9 May 2010, Sierau was re-elected with 43.8% of the vote for lord mayor of Dortmund. He was inaugurated on 18 May 2010. A change in the municipal code in 2007 and a concomitant extension of the term of the mayor had the consequence that the elections of the councils and district councils and the choice of headquarters officials fell apart (mayor, mayor and district administrators) in North Rhine-Westphalia. The newly elected state government of North Rhine-Westphalia in 2012 was created with the law to strengthen local democracy for some key administrative officials whose term ends before 20 October 2015, and the unique opportunity to resign prematurely in order already by 25 May 2014 to enable a common election date for the councils and headquarters officials. Based on the decision by Sierau in December 2009, to face the repeat election on 9 May 2010, whose term of office has been set for this election by individual decree of the Interior Ministry for six years. It ended, thus scheduled, on 18 May 2016. Therefore, Sierau did not fall under the provisions of the Act to strengthen local democracy. Sierau nevertheless decided to resign from his position as mayor with effect on 22 June 2014 so as to perform a common choice of local bodies and the Mayor of Dortmund on 25 May 2014. Since the provisions of the Act to strengthen local democracy not intervened in this case, a non-selection for loss of Sierau's pension entitlements from the time he would have led as mayor. The
Regierungsbezirk of
Arnsberg had raised no legal objections to a resignation and the renewed candidacy. He explained his decision with the strengthening of democracy by an expected higher turnout and cost savings of at least EUR 500,000 by the common election date. Sierau did not run for the local elections as a mayor in 2020. His successor will be
Thomas Westphal, who was elected in the runoff election on September 27, 2020. == Mandate and Functions ==