[34][42] On 30 January 2008, at 27, Ardern was elected president of the International Union of Socialist Youth (IUSY) at their world congress in the Dominican Republic for a two-year term until 2010.

[52] Ranked 5th on Labour's list, Ardern was still returned to Parliament where she became Shadow spokesperson for Justice, Children, Small Business, and Arts & Culture under new leader Andrew Little.

Ardern put forward her name for the Labour nomination for the Mount Albert by-election to be held in February 2017[56] following the resignation of David Shearer on 8 December 2016.

Under the country's mixed-member proportional (MMP) voting system, New Zealand First held the balance of power and chose to be part of a coalition government with Labour.

[88][89] The next day, Ardern indicated her intention to take ministerial responsibilities in the National Security and Intelligence; Arts, Culture and Heritage; and children's portfolios; reflecting the shadow positions she held as Leader of the Opposition.

[104] Among its provisions, the package gradually increased paid parental leave to 26 weeks and introduced a $60 per-week universal BestStart Payment for low and middle-income families with young children.

[110] Economically, Ardern's government has implemented steady increases to the country's minimum wage[111] and introduced the Provincial Growth Fund to invest in rural infrastructure projects.

[131] On 12 November 2017, Trade and Export Growth Minister David Parker and Ardern announced that the government would continue participating in the Trans-Pacific Partnership negotiations despite opposition from the Green Party.

[143][144] Her address to the General Assembly praised the United Nations for its multilateralism, expressed support for the world's youth, called for immediate attention to the effects and causes of climate change, for the equality of women, and for kindness as the basis for action.

In a statement broadcast on television, Ardern offered condolences and stated that the shootings had been carried out by suspects with "extremist views" that have no place in New Zealand, or anywhere else in the world.

"[157] Ardern received international praise for her response to the shootings,[158][159][160][161] and a photograph of her hugging a member of the Christchurch Muslim community with the word "peace" in English and Arabic was projected onto the Burj Khalifa, the world's tallest building.

[174] Alastair Campbell, a journalist and adviser in Tony Blair's British government, commended Ardern for addressing both the human and economic consequences of the coronavirus pandemic.

[175] In mid-April 2020, two applicants filed a lawsuit at the Auckland High Court against Ardern and several government officials including Director-General of Health Ashley Bloomfield, claiming that the lockdown imposed as a result of the COVID-19 pandemic infringed on their freedoms and was made for "political gain".

[200] In mid-December 2022, Ardern was recorded on a hot mic calling the leader of the ACT Party, David Seymour, an "arrogant prick" during Parliament's Question Time.

[201][202] The two politicians subsequently reconciled and joined forces to raise NZ$60,000 for the Prostate Cancer Foundation by auctioning a signed and framed copy of the Prime Minister's remark.

[207] On 17 December, Ardern also announced that the Government had purchased two more vaccines from the pharmaceutical companies AstraZeneca and Novavax for New Zealand and its Pacific partners in addition to the existing stocks from Pfizer/BioNTech and Janssen Pharmaceutica.

[212] In early October 2021, after one and a half years of pursuing a "Covid zero" strategy—a policy maintained even as many of its neighbors transitioned to living with the viral threat—New Zealand finally ended its zero-Covid approach, becoming the last country in the world to do so, apart from China.

[252][253] On 2 August, Ardern met with Fiame to discuss issues of concern to bilateral relations including climate change, economic resilience, COVID-19, health and Samoan seasonal workers in New Zealand.

[256][257] In mid-November 2022, Ardern attended the East Asia Summit in Cambodia where she condemned the Myanmar military regime's execution of political prisoners and called for consensus in response to the Russian invasion of Ukraine.

The opposition National and ACT parties' leaders Christopher Luxon and David Seymour thanked Ardern for her service while expressing disagreement with her government's policies.

Similar sentiments were echoed by the Māori Party's co-leaders Debbie Ngarewa-Packer and Rawiri Waititi, who praised her leadership qualities and contributions to New Zealand society.

[266][267] New Zealand First leader and former Deputy Prime Minister Winston Peters attributed Ardern's resignation to her government's failure to deliver on promises and targets during the 2020–2023 parliamentary term.

[267] Prominent New Zealanders, including actor Sam Neill, comedian and writer Michèle A'Court, and Internet entrepreneur Kim Dotcom, expressed gratitude for Ardern's service.

[268][269] In several opinion polls, Ardern's domestic popularity had reached all-time lows as of 19 January 2023, although she denied this would affect the Labour Party's chances of winning the next election.

She will also work with Harvard's Berkman Klein Center for Internet & Society as its first Knight Tech Governance Leadership Fellow during that period where she will focus on the study of online extremism.

[278][279] In mid June 2024, the Center for American Progress Active Fund announced that Ardern would be leading the Field Fellowship programme for training new "emerging leaders.

[311][312][313] After becoming the Labour Party leader, Ardern received positive coverage from many sections of the media, including international outlets such as CNN,[314] with commentators referring to a "Jacinda effect" and "Jacindamania".

In mid-May 2021, Fortune magazine gave Ardern the top spot on their list of world's 50 greatest leaders, citing her leadership during the COVID-19 pandemic as well as her handling of the Christchurch mosque shootings and the Whakaari / White Island eruption.

[340] In the 2023 King's Birthday and Coronation Honours, Ardern was appointed a Dame Grand Companion of the New Zealand Order of Merit (GNZM), for services to the State.

[346][347] The couple first met in 2012 when they were introduced by mutual friend Colin Mathura-Jeffree, a New Zealand television host and model,[348] but they did not spend time together until Gayford contacted Ardern regarding a controversial Government Communications Security Bureau bill.