After the 2020 French municipal elections, he became the first socialist Mayor of Nancy since the end of the second World War.

Mathieu Klein is a member of the Socialist Party since 1992, and has his support in favour of a positive result in the French referendum on the Maastricht Treaty.

In 2005, Klein contested a by-election in Meurthe-et-Moselle's 1st constituency, but was defeated by 399 votes by the incumbent Laurent Hénart.

[11][12] In the 2007 French legislative election, Klein stood as a candidate again and lost, obtaining 49.2% of the vote in the Second round.

[23][24] In 2016, Klein announced his support for Manuel Valls and became one of his spokespersons during the 2017 French Socialist Party presidential primary campaign.

He was confirmed as Mayor on 15 July 2020 by 43 votes in the municipal council of Nancy, becoming the first left-wing person to exercise this mandate since the end of the Second World War.