When carried out in the June 15, 2014 runoff he, then reached 51.6% of the vote, his opposition candidate Annette Littmann from the Christian Democratic Union of Germany, who received 32.0% in the first ballot, subjected to 48.4%.

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".

Within the SPD Dortmund developed in 2008 a discussion of possible candidates for the post of Mayor in the forthcoming local elections.

On 30 August 2009, Sierau had 45.5% of the vote, succeeding Gerhard Langemeyer [de] (SPD) as mayor of Dortmund.

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.

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.

[9] 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.