She was selected as leader in a unanimous vote this morning, and Te Tai Tokerau MP Kelvin Davis was chosen as her deputy unopposed - believed to be the first Maori to hold the position.
Her promotion to the top job came after former Labour leader Andrew Little announced he would stand down because of a string of poor poll results, saying the party "deserved better" and that it needed a campaign uncluttered by questions about leadership and polls.
Ardern held her first press conference as leader in the Parliament's stately legislative chamber, a space reserved for big political occasions. By her side was Davis and the party's front bench - Grant Robertson, Phil Twyford, Megan Woods, Chris Hipkins, Carmel Sepuloni, David Clark, David Parker and Stuart Nash.
"Everyone knows that I have just accepted - with short notice - the worst job in politics," she said.
"But I also welcome this job."
Ardern has previously sworn that she never wanted to become leader or Prime Minister, but said today that the party was faced with an extraordinary situation.
"The circumstances may not be what Labour had planned for this campaign. But that has not weakened my resolve or my focus. Or my team's resolve.
"We are determined and steadfast, and I consider myself incredibly lucky to now call myself leader, and equally fortunate to walk alongside someone as committed and talented as Kelvin and the rest of my team.
"This team is about to run the campaign of our lives."
Ardern said she stood by Labour's "broad vision" but she planned to "take stock" for 72 hours before revealing her changes to Labour's election campaign.
"You will see my personal stamp on this campaign. Relentless positivity will be amongst it."
She would not confirm whether Labour would reinstate a capital gains tax - a key Labour policy dropped by Little.
The Memorandum of Understanding with the Green Party would remain intact, though Ardern stressed that her focus was on Labour's campaign and no other.
Confident and wise-cracking, she batted away questions about the leadership change, her experience, and her ability to handle a squabbling left-wing bloc.
She accepted she was a "young proposition for the party" at 37 years old but said she had 20 years' experience "in and around politics", nine years as an MP, and a "pretty diverse rural background".
Just yesterday senior MP Stuart Nash said a new leader would be a disaster for the party so close to the general election. As Nash, red-faced, stepped forward to defend his comment, Ardern interjected: "Stuart has already acknowledged that he was wrong."
Asked if she had the ability to lead a government that included the Green Party and New Zealand First, she shot back: "I used to be the president of an international youth organisation that had members from Lebanon, Palestine and Israel. I think I can do this."
On whether she could get on with NZ First leader Winston Peters, she noted that they both had a taste for single malt whiskey.
Speaking about her new deputy, she said Davis' record stood for itself. A former teacher, Davis has made an impression in the Corrections portfolio, where he prompted an inquiry into the privately-run Mt Eden Prison and has defended the rights of Kiwis in offshore detention centres.
Davis said today he would reconsider his decision to remove himself from the party list for the general election - a move which was designed to bring more Maori Labour MPs into Parliament.
Labour's constitution requires the party's leader and deputy to be listed at one and two.
Little's demise began on Sunday when he publicly admitted he had discussed stepping down last week after a poor poll result - a suggestion that his caucus rejected.
Two further poll results confirmed that Labour had slid to a historic low of 24 per cent, further undermining Little's leadership.
Ahead of a caucus meeting this morning, Little suggested he would stare down any challenge to his leadership from his caucus. But by 10am, he had given that up and quit.
Ardern said she only found out about his plans on her taxi ride to Parliament this morning. She hasn't yet had time to tell her parents, who live in Niue, or her partner Clarke Gayford.
"Mum and Dad are going to get a surprise."
