[3] The DSS contested the 2000 Serbian local elections as part of the Democratic Opposition of Serbia (DOS), a broad and ideologically diverse coalition of parties opposed to Slobodan Milošević's government.
For the next four years, he held administrative responsibility for sports and youth, refugees and social issues, and relations with religious communities.
[13][14][15] The far-right Serbian Radical Party (SRS) won the election and formed a coalition government with the Strength of Serbia Movement (PSS), and the DSS served in opposition.
[20] In early 2005, Mladenović said that Sreten Lukić would surrender voluntarily to the International Criminal Tribunal for the Former Yugoslavia (ICTY) in The Hague to face charges of war crimes.
[21][22] The following year, he criticized the European Union's pressure tactics on Serbia to surrender prominent war crimes suspects such as Radovan Karadžić and Ratko Mladić, both of whose whereabouts were unknown at the time.
Mladenović said in June 2006, "What will [additional conditions on Serbia's bid for European Union membership] bring if Mladić is dead?
Mladenović responded, "Serbia has an interest in exposing and condemning all war crimes in the history of the former Yugoslavia, in which the Serbian people were the biggest victims.
[28] The following month, he described a revised version of the Ahtisaari plan as "worse than the previous one," in that it fully corresponded "with the stance of the Albanian separatists.
"[29][30] The 2007 Serbian parliamentary election did not produce a clear winner, and the DS, DSS, and G17 Plus ultimately formed an unstable coalition government with Vojislav Koštunica continuing in the role of prime minister.
[33] In November, he said that a new European Union (EU) mission to Kosovo and Metohija would be unacceptable for Serbia before a multilateral settlement on the status of the territory.
Mladenović blamed Tadić and the DS for this situation, stressing that the DSS proposal was a legitimate offer and not a form of political blackmail.
The DSS contested the election in a partnership with New Serbia (NS), and Mladenović received the sixty-second position on their combined list.
[40] The talks ultimately failed, and the Socialists instead entered a coalition government with the For a European Serbia (ZES) alliance led by the DS.
[49][50] In July 2008, Mladenović announced that the DSS would support a rally organized by the Radical Party against the recent arrest of Radovan Karadžić.
"[54] In June 2009, he said that the DSS was prepared to align itself with the recently established Serbian Progressive Party (SNS) to form new coalition governments in the Belgrade municipalities of Zemun and Voždovac.
[62][63] The Serbian Progressive Party (SNS) and its allies won a majority victory with sixty-three seats out of 110 and afterward formed a new coalition government that included the DSS.
Mladenović also received the tenth position on the DSS's list in the 2014 Serbian parliamentary election, which was once again held concurrently with the Belgrade vote.
[67] Mladenović subsequently formed the Independent Democratic Party of Serbia and created alliances with the SNS at the republic and city levels.
[a][72] He resigned his seat on 3 October 2016 as he could not hold a dual mandate as a national assembly member and deputy mayor of Belgrade.
[75] He served as acting mayor of Belgrade from 28 May to 7 June 2018, following Siniša Mali's resignation to accept a cabinet position and before Zoran Radojičić was confirmed as his replacement.