Horner has lived in Dublin's North Inner City for over a decade having previously lived in England, Uganda and India working in politics, human rights and community development projects. She holds a degree in Anthropology and Development Studies and a Masters in Politics and Public Policy. She is a trade union activist and is a member of SIPTU's National Equality and Global Solidarity Committees.[4] Horner campaigned with Together for Yes in the 2018 referendum to ratify the Thirty-sixth Amendment, which removed the constitutional ban on abortion in Ireland.[5]
Horner was co-opted to the Dublin City Council seat held by Ciarán Cuffe in September 2019 following Cuffe's election to the European Parliament the previous May.[6] She was re-elected to the council in the 2024 local elections winning 9.5% of the first preference votes in the North Inner City local electoral area.[7] During the 2024 election campaign, Horner was assaulted and received death threats while erecting posters on North Circular Road, Dublin.[8]
In May 2025, it was announced that she had been elected to succeed Pauline O'Reilly as chair of the Green Party. In February 2026, the party announced that Horner would be the party's candidate in the 2026 Dublin Central by-election.[9]
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