Steiner joined the Hungarian Socialist Workers' Party (MSZMP) in 1986, then became a founding a member of the Socialist Party in October 1989. He served as chairman of the party's national ethics committee between 1998 and 2004. Steiner became a Member of Parliament from his party's Budapest regional list during the 2002 parliamentary election. He was involved in the Committee on Constitutional Affairs and Justice. He was re-elected in 2006 and 2010 too from the MSZP regional list both times.[2]
He was elected mayor of the downtown of Budapest (5th district) in the 2002 local election. He held the office until the 2006 local elections. Then he became a member of the General Assembly of Budapest, where he functioned as leader of the MSZP caucus until 2010.[2] He was president of the Central Hungary Development Council between 2008 and 2010.[1]