"We knew that Jacinda was the favourite to win and we were standing to talk about our issues and to demonstrate how we can work positively with Labour. I think it's a great result for Jacinda and well deserved," she said.
"You'd always like a few more votes but we're very happy with the campaign we ran."
Voter turnout for the by-election was low. There are 55,520 people eligible to vote in the Mt Albert electorate and 46,958 -- or 84.6 per cent -- are enrolled. Just 12,971 -- or 27.6 per cent of those enrolled -- voted.
Ardern was always heavily favoured to win the seat.
National did not stand a candidate, with Prime Minister Bill English saying they wanted to focus on September's general election.
Thirteen candidates competed for David Shearer's old electorate after the former Labour MP and party leader resigned in December to take up a United Nations posting in South Sudan.
Shearer won Mt Albert in 2014 with a majority of 10,656 votes, or about 59 per cent. In the 2009 byelection that saw him enter Parliament Shearer got 63 per cent of the vote.
National got 14,360 party votes in the seat in 2014, ahead of Labour on 10,823 and the Green Party at 8005.
National's decision not to stand in Mt Albert came after its candidate Parmjeet Parmar lost heavily to Labour's Michael Wood in the Mt Roskill by-election in December.
The Green Party did not stand in that by-election, a decision reached after Labour and the Greens signed a Memorandum of Understanding, with the aim of changing the Government.
Ardern is a list MP and her victory means another Labour List MP will come into Parliament.
Ardern recently moved into Mt Albert with her partner Clarke Gayford but had been intending to stand in Auckland Central for the third time prior to Shearer's resignation.