Today Key thanked supporters for helping "get New Zealand back on its feet".
He also praised his successors saying he had enormous faith in the leadership of the National Party.
"It has been an absolute honour to serve in Parliament since 2002, as MP for Helensville, National Party Leader, and Prime Minister," Key said.
"One of the great privileges of my political career and my life was to meet so many hard-working and inspiring New Zealanders. I remain as ambitious for them, and New Zealand, as the day I entered Parliament.
"I would like to thank all those who backed me and the National-led Government to build a stronger and more resilient country. We got New Zealand back on its feet, got people into jobs, got back into surplus and tackled natural disasters.
"I have enormous faith in the leadership team of Bill English and Paula Bennett to provide the stability and continuity we need to build on that strength, while continuing to support those in need."
His family also came in for praise.
"I would also like to take this opportunity to thank my wife, Bronagh, and my children Stephie and Max. I absolutely could not have done this job without their ongoing love and support."
Early departures such as Key's require the consent of Parliament and at least a 75 per cent majority but that will be sewn up easily with Labour's support.
Former Labour leader and New Lynn MP David Cunliffe has also announced his retirement from politics and has his party's blessing to leave early, along with Key.
Cunliffe, who will give his valedictory speech on April 11, said it had been an enormous privilege to serve as a politician and he had loved "nearly every minute of it".
"In my time as MP for Titirangi from 1999-2002, as MP for New Lynn since then; and as a Minister of the Crown, senior Opposition spokesperson and as Leader of the Opposition, I have never forgotten the reason I came into politics and the reason I'm passionate about the Labour Party," said Cunliffe.
"I've loved representing my electorate of New Lynn and the vibrant communities within it. I'm deeply grateful to my wonderful staff and members of the awesome New Lynn Labour Electorate Committee, who have stood solidly behind me through the ups and downs."
He said as a cabinet minister he had the chance to do things that he hoped would continue to make a difference, including improvements in telecommunication, better healthcare planning, more humane and focused immigration, better jobs in the regions, and a fairer tax system.
Labour and National's combined vote of 91 MPs amounts to just more than 75 per cent.
But it is unlikely that any of the smaller parties would be churlish enough to oppose their early departure, if it is not a mass exodus.
Long-serving Pakuranga MP Maurice Williamson is expected to join the early departures, anxious as he is to take up his first job after politics as New Zealand Consul-General in Los Angeles.
It is not yet known what work Key is headed to but he has talked about taking directorships.
He is likely to be in high demand having led a Government for eight years through the global financial crisis with more success than most.
He also expanded his contacts globally, making close connections with leaders such as Barack Obama, Angela Merkel, Xi Jinping, David Cameron and Stephen Harper.
He already has an extensive network of contacts in business and the financial markets through his stellar career as a trader and executive with Merrill Lynch.
The other National MPs retiring this year are Murray McCully (East Coast Bays), Chester Borrows (Whanganui), Craig Foss (Tukituki), and Peseta Sam Lotu-Iiga (Maungakiekie).
List MPs who have given notice of their intention to retire this year include Education Minister Hekia Parata who is due to relinquish her portfolio to Nikki Kaye in May; Labour MP Clayton Cosgrove and Green MPs Catherine Delahunty and Steffan Browning. If they leave before the election, they do not require the permission of Parliament. They would be replaced by the next person on their party list.
Key's resignation was for no apparent single reason except he wanted to leave on his own terms and while he was still extraordinarily popular.
He recommended that his deputy and finance minister Bill English be elected by the National caucus to replace him, which duly happened.
In the first political poll of the year, English rated highly, 31 per cent, as preferred prime minister, against the poll of Labour leader Andrew Little on 7 per cent.
