Since the start of the campaign, and especially in May 2021, polling for mayor had consistently placed left-wing social activist and environmental researcher, Tomislav Tomašević, clearly in the lead, with 30–40 percent of the vote (as was also the case with his Green–Left Coalition in the Assembly election).

Meanwhile, the candidates of Croatia's two major parties – the HDZ and the SDP, were well behind, as were conservative musician-turned-politician Miroslav Škoro, acting mayor Jelena Pavičić Vukičević and former Minister of Justice Vesna Škare-Ožbolt.

[12] The first round of the mayoral election, held on 16 May 2021, was convincingly won by leftist candidate Tomislav Tomašević (with 45.15%), while right-wing nationalist Miroslav Škoro finished a distant second (with 12.16%).

Namely, a major earthquake occurred in the city on 22 March 2020 and caused extensive damage to historic buildings.

[26] Other major campaign issues include the troubling state of the city's finances,[27][28] its allegedly oversized bureaucratic apparatus,[29] as well as the corruption, nepotism and cronyism which took place during the Bandić administration.

Austrian Der Standard wrote that Škoro's statements polarise the right-wing voters and could cause some to turn against him in the runoff.

[33] The Independent and German Frankfurter Allgemeine Zeitung described the unexpectedly high vote shares attained by Tomašević and We can!

[34][35] The Frankfurter Allgemeine Zeitung wrote that "anything but a new victory for [Tomašević on 30 May] would be a sensation", and called the Croatian local elections a disaster for Croatia's biggest left-wing party, the SDP, which won just 8.9 percent of the vote for the Zagreb Assembly and generally performed poorly across the country.

in the national political arena, should their campaign promises be executed and should the fight against Bandić's "corruption and clientelism" succeed.

On 30 April 2021, the State Electoral Commission (SEC) announced that 10 candidates had presented enough valid signatures from registered voters in order to be placed on the ballot in the elections for the mayor of Zagreb.

In order to qualify for legislative representation, a party or coalition must receive at least 5% of the number of valid votes cast in an election.

In this context, mayor Tomislav Tomašević, as well as his two deputies, Danijela Dolenec and Luka Korlaet, were ineligible to take up their seats in the Assembly, and were replaced by Marija Krnić, Sanja Bilas and Ana Profeta, respectively.

[82][83] Akter Public was accused by several candidates of attempting to influence voters to vote for Miroslav Škoro in an illegal phone campaign.

The phone number from which the illegal campaign was carried out was registered to Akter Public, according to the Croatian Regulatory Authority for Network Industries.