From 2004 to 2006 he served as State Secretary in the Ministry of Youth, Family, Social Affairs and Equal Opportunities during the rule of Prime Minister Ferenc Gyurcsány.

As chairman of MSZP in the 2010 municipal elections he sang "bikicsunáj", while also using the term "comrades" (elvtársak) to address other party members and sympathizers.

[4] During his leadership, Mesterházy stabilized the party which was "in ruins" after the supporters of Ferenc Gyurcsány left MSZP to form Democratic Coalition (DK).

However, there were also criticisms that Mesterházy, similar to structure of the right-wing Fidesz, over-centralized the party and overshadowed his internal opposition and former influential leaders, such as László Kovács, Ildikó Lendvai and Imre Szekeres.

According to the critics, he set up the parliamentary group with own supporters, who occupied the most important positions within the party.

Following a months of wrangling between Gordon Bajnai and him, Mesterházy was elected candidate for Prime Minister of the Unity electoral alliance, but his movement failed to win and suffered the largest defeat.

[7] After the obvious failure, Mesterházy and the entire presidium of the Socialist Party tendered their resignation.

As a member of the consultative interparliamentary organisation since 2006, Mesterházy was elected head of the NATO Parliamentary Assembly's Socialist group in November 2018.