Ardern became New Zealand's third female Prime Minister after Jenny Shipley (1997–1999) and Helen Clark (1999–2008). She is a member of the Council of Women World Leaders. At aged 37, Ardern is also the youngest person to become New Zealand's head of government since Edward Stafford, who became premier in 1856.
On 19 January 2018, Ardern announced that she was pregnant, and that Winston Peters would become acting Prime minister for six weeks after the birth. Following the birth of a daughter, she took her maternity leave from 21 June to 2 August 2018.
Ardern focused on cutting child poverty in New Zealand by half within a ten years. In July 2018, Ardern announced the start of her government's flagship Families Package. The package would increase paid parental leave to 26 weeks and introduced a $60 per-week universal BestStart Payment for low and middle-income families with young children. In 2019, the government a school lunches pilot programme to help cut child poverty numbers. She also increased welfare benefits by $25, expanding free doctor's visits and giving free menstrual hygiene products in schools.
Ardern's government added increases to the country's minimum wage over a period of time. Her government also cancelled the National Party's planned tax cuts, saying instead it would focus on healthcare and education. The first year of post-secondary education was made free on 1 January 2018 and the government agreed to increase primary teachers' pay by 12.8 (for beginning teachers) and 18.5 per cent by 2021. Even thought the Labour Party campaigned on a capital gains tax for the last three elections, Ardern promised in April 2019 that the government would not pass a capital gains tax under her leadership.
On 2 February 2018, Ardern went to Waitangi for the annual Waitangi Day commemoration and she stayed in Waitangi for five days. Ardern became the first female Prime Minister to speak from the top marae. Her visit was popular among Māori leaders, with commentators seeing the difference between her performance to the Prime Ministers before her.
In September 2019, she was criticised for her handling of an allegation of sexual assault against a Labour Party staffer. Ardern said she had been told the allegation did not involve sexual assault or violence before a report about the incident was published in "The Spinoff" on 9 September. Many did not believe this as one journalist found that Ardern's claim was "hard to swallow".
Ardern does not support putting people who use cannabis in prison and promised to hold a referendum on the issue. A non-binding referendum to legalise cannabis was held at the same time of the 2020 general election. Ardern said she used cannabis when she was young during a televised debate before the election. In the referendum, voters did not support the proposed cannabis legalisation and many found that Ardern not supporting the 'yes' campaign may have caused the legalisation's defeat.
On 5 November 2017, Ardern made her first official trip to Australia, where she met Australian Prime Minister Malcolm Turnbull for the first time. Relations between the two countries had not been good because of Australia's treatment of New Zealanders living in the country, and shortly before taking office. Turnbull said of Ardern: "we trust each other...The fact we are from different political traditions is irrelevant".
In November 2017, Ardern announced that the government would being part of the Trans-Pacific Partnership negotiations even though the Green Party did not support it. On 25 October 2018, New Zealand ratified the Comprehensive and Progressive Agreement for Trans-Pacific Partnership, which Ardern had described as being better than the original TPP agreement.
In December 2017, Ardern supported for a United Nations resolution criticising U.S. President Donald Trump's decision to recognise Jerusalem as the capital of Israel.
On 24 September 2018, Ardern became the first female head of government to go to the United Nations General Assembly meeting with her baby present. Her address to the General Assembly on 27 September had positive support from the United Nations where she talked about climate change, for the equality of women, and for kindness.
In October 2018, Ardern criticized China for its Xinjiang re-education camps and human rights abuses against the Uyghur Muslim minority in the country. Ardern was also worried about the treatment of the Rohingya Muslims in Myanmar. In November 2018, she met with Myanmar's leader Aung San Suu Kyi and offered any help New Zealand could give to solve the Rohingya crisis.
On 23 September 2019, at a United Nations summit in New York City, Ardern had her first formal meeting with U.S. President Donald Trump. She said that Trump showed "interest" in New Zealand's gun buyback policy.
On 28 February 2020, Ardern criticised Australia's policy of deporting New Zealanders, many of which had lived in Australia but had not become Australian citizens.
On 15 March 2019, 51 people were fatally shot and 49 injured in two mosques in Christchurch. In a statement broadcast on television, Ardern said 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. She also described it as a well-planned terrorist attack.
She announced a period of national mourning and created a national condolence book that she opened in the capital, Wellington. She also travelled to Christchurch to meet first responders and families of the victims. In an address at the Parliament, she said that she would never say the name of the attacker: "Speak the names of those who were lost rather than the name of the man who took them ... he will, when I speak, be nameless". Ardern became popular in the country and the world for her response to the shootings. 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. A mural of this photograph was opened in May 2019.
In response to the shootings, Ardern announced her government's plans to push for stricter gun laws. She said that the attack had shown many weaknesses in New Zealand's gun law. On 10 April 2019, less than one month after the attack, the New Zealand Parliament passed a law that bans most semiautomatic weapons and assault rifles, parts that turn guns into semiautomatic guns, and higher capacity magazines.
On 15 May 2019, Ardern and French President Emmanuel Macron co-chaired the Christchurch Call summit for the purpose to bring together countries and tech companies to end the ability to use social media to organise and promote terrorism and violent extremism.
On 14 March 2020, Ardern announced the government's response to the COVID-19 pandemic in New Zealand saying that the government would be requiring anyone entering the country from midnight 15 March to isolate themselves for 14 days. She said the new rules will mean New Zealand has the "widest ranging and toughest border restrictions of any country in the world". On 19 March, Ardern said that New Zealand's borders would be closed to all non-citizens and non-permanent residents, after 11:59 pm on 20 March (NZDT). Ardern announced that New Zealand would move to a nationwide lockdown, at 11:59 pm on 25 March.
National and international media had positive responses about her leadership and quick response to the outbreak in New Zealand. "The Washington Post" said her interviews, press conferences and social media is a "masterclass in crisis communication". In mid-April 2020, two people sued Ardern and other government officials, saying that the lockdown limited their freedoms and was made for "political gain". The lawsuit was dismissed.
On 5 May 2020, Ardern and Australian Prime Minister Scott Morrison announced plans to create a trans-Tasman COVID-safe travel zone that would allow residents from both countries to travel freely without travel restrictions.