However she eventually decided to go.
Although Little had asked her to remain in the health portfolio and on the front bench, King decided she would leave Parliament at the election.
She had already decided not to stand in her Rongotai electorate.
Little said Ardern was elected deputy with unanimous support from MPs.
"I'm grateful caucus has recognised her ability to help Labour communicate our vision for a better New Zealand.
"Jacinda's a fierce advocate on the issues she has championed, such as child well-being. She's worked hard during her time in Parliament and has the experience to be an excellent deputy leader.
"We're going to make a great team on the campaign trail. Jacinda's a driven and dynamic campaigner who reaches a generation of New Zealanders neglected by this government."
Ardern said she was grateful for the support from caucus and was "buoyed by the support the team has shown me for this role".
She had spoken to each MP before the vote.
She said Labour had more to offer the younger generation.
"[National] has basically said anyone born from 1972 will be paying the cost for Bill English not putting in contributions to the Super Fund."
"He's effectively said that it's my generation that now has to pick up the tab."
She said that was the same generation that had to pay for their own tertiary education through student loans.
Little said he was not concerned that Ardern's popularity would see her overtake him as preferred Prime Minister.
"These only one leader of the party; that's me."
Ardern said she did not believe she would overtake Little as preferred PM.
"I'm here to see Andrew Little elected as PM and nothing else. My job is to support him and that's what I'm going to do."
