Ms Ardern paid tribute to him. "It's important that National's new leader has the time and the best possible opportunity to achieve that".
"I don't think New Zealand will ever appreciate the depth of his thinking".
"A world of gratitude also needs to go to his wife Mary and all of his children for sharing their husband and father with us over all these years".
"My view about what you do in politics is that you need to be satisfied yourself about what you do".
English led the National Party into last September's election, where it fell five seats short of a majority.
Mr English said being leader had been a "huge privilege".
"Our team has the talent, the ideas and the energy to return to government in 2020", English told a press conference Tuesday in Wellington.
New Zealand National Party leader Bill English announced he is bowing out of politics, drawing the curtain on a 27-year career that included eight years as finance minister and nine months as prime minister.
"The tax cuts implemented by Bill English in 2010 were significant and working New Zealanders continue to reap the benefits today".
The decision followed a meeting with the party's caucus in the morning.
A major source of concern was the performance and personality of deputy leader Paula Bennett and Mr English's antagonistic relationship with Winston Peters, whom he'd pushed to the brink, making it all-but impossible to form a Coalition government.