Colau was one of the founding members and spokespeople of the Plataforma de Afectados por la Hipoteca (PAH) (Platform for People Affected by Mortgages), which was set up in Barcelona in 2009 in response to the rise in evictions caused by unpaid mortgage loans and the collapse of the Spanish property market in the wake of the 2008 financial crisis.

Barcelona en Comú won a plurality in the elections (11 of 41 city council seats) and on 13 June 2015 she was sworn in as mayor with the favourable vote of an absolute majority of councillors.

[21] During her period as mayor of Barcelona, Colau has maintained a political stance against activities that are susceptible of contributing to greenhouse gas emissions and air pollution.

She has repeatedly opposed the expansion of El Prat airport[22] and the use of private cars in the city,[23] and has pushed regional authorities to restrict the number of cruise ships arrivals in Barcelona.

[24] In 2020 she declared a "climate emergency", advocating limiting the consumption of meat at schools and forbidding councillors from using the Barcelona-Madrid air shuttle.

[25][26][27] Colau also called for a reduction of air traffic during the C40 Cities 2019 summit, arguing that aeroplanes generate greenhouse gas emissions that are "very dangerous for the planet".

During her second mandate she kickstarted the union of the two tramway networks (Trambaix and Trambesòs) along Avinguda Diagonal,[30] with the first section reaching Verdaguer (from Glòries) being opened in November 2024.

[non-primary source needed][36] Ada Colau was indicted by the Spanish judiciary in 2022, following a complaint from the Vauras fund, for alleged irregularities in the handing of subsidies to entities linked to her party Barcelona en Comú, including to the Platform for People Affected by Mortgages which she founded in 2009.

The magistrate points to the fund Vauras for using a complaint against the former mayoress for "purposes unrelated to those of the criminal proceedings" because "the housing policies" promoted by the City Council "were not favorable to her".

[46] In 2022, the Ethics and Conduct Committee of the Council of Barcelona produced a report in which it heavily criticised the decision to hire Alicia Ramos, partner of the Housing councillor, as an advisor to the staff of Ada Colau on grounds of a potential conflict of interest.

[47][48] Colau has also been criticised for several other controversial hirings, such as that of her husband Adrià Alemany as an advisor to Barcelona en Comú, the party she leads.