"The whole world can see why I've never had a moment's regret. Proud to keep moving with you boss," he posted today.
Just six weeks out from the general election and with Labour flagging in the polls, the new leadership prompted an unimaginable change in the party's fortunes.
After talks with New Zealand First, Labour formed a coalition Government. The Greens provide confidence and supply.
It's been a tumultuous first term for Ardern, including the March 2019 Christchurch mosque terror attacks, last year's White Island eruption and now the Covid-19 pandemic.
Throughout each crisis she has won praise from those personally affected as well as observers across the globe.
Lauded for her compassion and empathy, an iconic image of her comforting a woman was projected onto the world's tallest tower, the Burj Khalifa, after the mosque massacre.
As the world's youngest leader at 37, she also became the world's second elected head of government to give birth during office. Her daughter, Neve, was born on June 21, 2018.
Last week, partner Clarke Gayford posted his adoration for his fiancee when she turned 40, wishing happy birthday to his "favourite achiever in the whole wide world".
This week she was dubbed the most eloquent leader in the world and is now being considered as one of the three most likely candidates for this year's 2020 Nobel Peace Prize.
