The 38-year-old, who gave birth while in office, could be seen playing with her three-month-old child before giving a speech at the Nelson Mandela peace summit in New York.
Ms Ardern’s partner Clarke Gayford held the baby Neve Te Aroha on his lap while the world leader addressed the UN’s General Assembly.
Mr Gayford – a fishing television presenter – is the main caregiver for the daughter and travelled with Ms Ardern to the US city to look after the baby.
He shared a photo of the child’s mocked-up UN diplomatic photo ID that called her "New Zealand first baby."
“Because everyone on twitter's been asking to see Neve's UN id, staff here whipped one up,” he tweeted.
“I wish I could have captured the startled look on a Japanese delegation inside UN yesterday who walked into a meeting room in the middle of a nappy change. Great yarn for her 21st.”
Because everyone on twitter's been asking to see Neve's UN id, staff here whipped one up.— Clarke Gayford (@NZClarke) September 24, 2018
I wish I could have captured the startled look on a Japanese delegation inside UN yesterday who walked into a meeting room in the middle of a nappy change.
Great yarn for her 21st. pic.twitter.com/838BI96VYX
UN spokesperson Stephane Dujarric heaped praise on Ms Ardern.
“Prime Minister Ardern is showing that no one is better qualified to represent her country than a working mother. Just five per cent of the world’s leaders are women, so we need to make them as welcome here as possible,” he said.
Samantha Power, a former US Ambassador to the UN under former President Barack Obama, tweeted: "I cannot stress how much the UN - and the governments that comprise it - need this."
The PM is continuing to breastfeed Neve - meaning the baby had to travel with her for the six-day trip.
Last week the rules in New Zealand were adjusted to allow the prime minister or ministers to travel with a nanny on overseas jobs and for the taxpayer to cover this.
But Ms Ardern said she would use her own money to pay for her partner’s tickets to New York and expenses as there were few official spousal engagements Mr Gayford would be obliged to go to and the majority of his time would be spent caring for their daughter.
In June Ms Ardern became the second world leader to give birth in office – taking six weeks of parental leave from work. On returning to work, she noted she was “very, very lucky” and in a “privileged” situation in comparison with most women in New Zealand.
Ms Ardern used her debut speech at the UN to draw attention to former South African leader Mr Mandela's "profound impact" on her country.
Her fellow panellists remarked on how peaceful Neve was waiting backstage with her father but the PM quipped she had not been at “3.30 this morning”.
“I have the ability to take my child to work, there’s not many places you can do that. I am not the gold standard for bringing up a child in this current environment because there are things about my circumstances that are not the same,” Ms Ardern said.
She drew attention to the unique conditions which mean she is able to raise her daughter while continuing to be Prime Minister – an arrangement which has often said she wants to be common for women who want to juggle their work with being a parent.
“If I can do one thing, and that is change the way we think about these things, then I will be pleased we have achieved something,” she said.
Ms Ardern became her party's youngest leader last year and NZ’s youngest in 150 years after beating former Prime Minister Bill English. This was the Labour Party’s first win in nine years.