He would leave parliament altogether at the end of the month, after 27 years, to "embark on new personal and professional challenges".
"I don't think New Zealand will ever appreciate the depth of his thinking".
"We wish Mr English all the best for his career after politics". Joking, he says he will enjoy his colleagues coming to him to ask for his vote.
Mr. English, also a former finance minister, led National to an election in September a year ago that eventually ended the party's almost decadelong incumbency.
"Our team has the talent, the ideas and the energy to return to government in 2020", English told a press conference Tuesday in Wellington.
"Mr English has held many roles during that time - constituent MP, party leader, minister and Prime Minister - and should be congratulated for both his commitment and perseverance".
The decision followed a meeting with the party's caucus in the morning.
He outpolled Ms Ardern in the popular vote but she forged an alliance with the Greens and maverick lawmaker Winston Peters to form a coalition government.