Former prime minister Helen Clark hopes to take up her new role as administrator of the United Nations Development Programme in April after the position was unanimously confirmed by the UN General Assembly on Wednesday.
Miss Clark says she applied for the job after seeing an advertisement in a London newspaper while on holiday in Britain in January.
It is the highest diplomatic position any New Zealander has ever held and the third highest position in the UN.
Miss Clark will take responsibility for the development programme's $NZ9 billion budget and oversee nearly 8000 staff in 166 countries.
The programme focuses on the elimination of poverty, good governance, health and conflict resolution.
She will also have the challenging job of making sure the world's wealthiest countries keep up their aid contributions amid the economic crisis.
Miss Clark will be based at the UN's headquarters in New York during her four-year term.
She says the position has been vacant since last March and the UN wants her to begin on 20 April.
Miss Clark says her networks in the international community would have played a big part in her selection and her application had international support.
She says there are many practical things to sort out before she takes up the job, including obtaining a UN passport and changing her visa status for visits to the United States.
Miss Clark has been a Labour Party MP since 1981 and was Prime Minister from 1999 to 2008. She says expects a by-election for her Mt Albert seat to be held about June.
Standing ovation in Parliament
Mr Key told MPs in Parliament on Wednesday that all of New Zealand can be proud of Miss Clark's appointment.
Mr Key says her appointment was supported by leaders from around the world.
"But let me be plain, Mr Speaker, in the end their support did not come because of the advocacy of the New Zealand Government.
"Ultimately, the reason Helen Clark won leaders' support for her bid, and indeed this role, was because of the personal abilities she bought to the table."
MPs gave Miss Clark a standing ovation in the House on Wednesday.