She holds a degree in Business Studies and started her political career as a member of the Basque Parliament, from 2001 to 2011.

After the Basque National Party's election victory in 2015, she was appointed president (speaker) of the Assembly for the 2015–2019 term.

She has been a commentator in radio and TV shows of EITB (Mezularia, Faktoria, Debatea, Egun on Euskadi), as well as a member of the General Assembly of Kutxa and a contributor in the press.

Before going into politics, she worked as a sales agent in a logistics and international transport company based in Irun, from 1997 to 2001.

Mendoza contested the 2011 Gipuzkoa foral election as the leading candidate of the Basque National Party in the Bidasoa-Oiartzun constituency.

She was the first vice president of the General Assembly of Gipuzkoa (the legislative body) during the 2011–2015 term, as well as the spokesperson of the Social Policy Committee.

She was a leading member of her party, tasked with the coordination of the parliamentary group, as well as EAJ-PNV's spokesperson on social policy issues.

[7] The reshuffle raised Mendoza's profile in government, and various media outlets presented her as potential successor to Olano, who had previously said he would not run for another term in office.

In Autumn 2022, as the Basque National Party was about to begin the process to select the leading candidates for the 2023 foral elections, media speculation grew regarding Mendoza's candidacy.

[10] In the 2023 foral election, the Basque National Party led by Mendoza obtained 32,04% of the vote and 17 seats in Gipuzkoa.

Nonetheless, the election devolved a hung parliament and the party announced that Mendoza would try to secure a majority in the investiture session.

[11] On 29 June 2023, Mendoza secured a plurality of votes in the investiture session and was elected president of the Gipuzkoa Provincial Council, becoming the first woman to hold the post.