Local government, 2019–2025
Leoni was elected to the Waitematā Local Board in 2019 on the City Vision ticket. She served as deputy chair for the first half of the term. On 23 February 2020, Leoni was selected as Labour's candidate for the Waikato electorate for the 2020 election, officially launching her campaign in August. Despite a strong pro-Labour swing, Leoni lost to the incumbent National MP Tim van de Molen. Her list placement of 66th was not high enough to enter parliament.
In the 2022 local body elections, Leoni was elected as a councillor for the Whau ward, after narrowly winning the election from the incumbent councillor Tracy Mulholland. Leoni was the first female Māori councillor to be elected to the Auckland Council. She was sworn in alongside the rest of the council on 28 October.
In 2024, Leoni announced that she would run for Mayor of Auckland in the 2025 elections. She received 22.16% of the vote, placing second to incumbent mayor Wayne Brown. She did not seek re-election to her council seat, and was succeeded by Sarah Paterson-Hamlin.
2026 general election and withdrawal
In March 2026 Leoni was selected to be the Labour Party candidate in the Tāmaki Makaurau electorate at the 2026 general election.
During an election debate hosted by Whakaata Māori (Māori Television) on 21 October 2026, Te Pāti Māori's Tāmaki Makaurau candidate and MP Oriini Kaipara accused Leoni of attending an election hui (meeting) at a Mongrel Mob gang pad the week before. When Leoni denied attending the event, Kaipara pointed out that there was a social media post confirming her presence at that event, which had since been deleted. Leoni and Kaipara were in a tight race for the Tāmaki Makaurau electorate, with the latter leading by two points in a recent Whakaata Māori-Curia poll for that electorate. On 22 October Te Ao Māori News released a video showing Leoni attending a Mongrel Mob gathering on 3 October in Papakura, Auckland. During the visit, she wore Labour campaign branding and urged attendees to vote. During the election debate the previous day, Leoni acknowledged she had relatives who were gang members. In response, Labour Party leader Chris Hipkins said that he accepted Leoni's account that she had not visited the gang pad for electoral campaigning purposes. Hipkins had earlier criticised Green and Te Pāti Māori candidates for meeting with members of the Mongrel Mob. Later that day, Leoni said she had attended a family function with some relatives who were gang members but denied visiting a gang pad during the 2026 election campaign.
On 23 October, National Party MP Paul Goldsmith criticised Leoni for socialising with gang members while wearing a party-branded jacket, saying it was an "appalling look" for the Labour Party and Mongrel Mob to be seen fundraising for each other. That same day, Leoni resigned as a Labour candidate. Despite initially supporting Leoni, Hipkins said that footage of her campaigning alongside gangs was "inconsistent with the standards" expected for Labour candidates. While acknowledging that one could not choose their family, Hipkins said he "firmly believe people should be supported to leave gangs and turn their lives around." Since nominations for the 2026 general election have closed, Leoni's name remains on the ballot, but she is no longer endorsed by the Labour Party. Following her resignation, Leoni issued a statement stating that she had resigned due to the "intense focus on her whanau and community."
On 26 October, Leoni confirmed that she would remain in the Tāmaki Makaurau race but would sit in Parliament as an independent candidate if elected during the 2026 general election.
