Footage showed Ardern waving to a cheering crowd as she left the building, flanked by security.
Ardern hugged and spoke to a number of supporters, thanking people as she went, before giving a final wave and climbing into a car.
Hipkins, 44, has promised a back-to-basics approach focusing on the economy and what he described as the “pandemic of inflation.”
He will have less than nine months before contesting a tough general election, with opinion polls indicating his Labour Party is trailing its conservative opposition.
New Zealand Governor-General Cindy Kiro officiated the swearing in ceremony after earlier accepting Ardern’s resignation.
Hipkins served as education and police minister under Ardern. He rose to public prominence during the COVID-19 pandemic, when he took on a kind of crisis management role.
But he and other liberals have long been in the shadow of Ardern, who became a global icon of the left and exemplified a new style of leadership.