Mr Key today signed agreements with ACT, the Maori Party and United Future giving his National government 70 votes on confidence and supply in the incoming 122-member Parliament.
In return he handed out five ministerial roles and a raft of minor policy concessions.
ACT and United Future had already pledged their support during the election campaign, but in a deal that was virtually unimaginable just three years ago Mr Key also succeeded in signing up the support of the Maori Party.
Under former leader Don Brash, National campaigned hard against the Maori seats and derided targeted spending on Maori as favouritism.
Today Mr Key clinched the deal by doing a U-turn on National's opposition to the Maori seats, saying it would be too divisive to scrap them without the agreement of Maori.
Mr Key described the agreement as historic.
"The Maori Party agreement is very much about inclusion. As I said on election night we will only succeed if we have all New Zealanders rowing in the same direction.
"The government I lead will be a government of opportunity -- opportunity for all New Zealanders regardless of their backgrounds, ethnicity or current circumstances."
Under the deal a group will be formed to look at constitutional issues including Maori representation, there will be a review of the Foreshore and Seabed Act and Maori Party co-leaders Pita Sharples and Tariana Turia will be ministers outside Cabinet.
Dr Sharples will pick up the Maori affairs portfolio, associate corrections and associate education, while Mrs Turia gets the community and voluntary sector, associate health and associate social development.
The round of deals also sees ACT leader Rodney Hide and his deputy Heather Roy installed as ministers outside Cabinet.
Mr Hide takes local government, the new portfolio of regulatory reform and associate commerce while Mrs Roy picks up consumer affairs and associate education and defence roles.
United Future leader Peter Dunne retains his revenue and associate health ministerial roles.
But in terms of policy all three support parties' agreements are thin on detail and immediate concessions. There is little cost to the new government in dollar terms.
In most cases where policy differs from National's there is agreement to set up a review, taskforce or committee to look at the issue.
Mr Key denied National was kicking difficult issues for touch and said the agreements were the start of a long-term process of co-operation.
But the lack of clarity could create headaches for Mr Key, with Mrs Turia today already indicating she would be unhappy if one of the private sector-led taskforces on government spending agreed with ACT and poked its head into her areas of ministerial responsibility.
"I can't imagine anybody from the private sector to come into an office and go through the budget line by line to be able to tell us what we should or should not be doing."
Mr Key smoothed over the tensions, saying spending reviews would be targeted at particular areas rather than across the board.
Labour leader Phil Goff today welcomed the new government, but said Mr Key would have to manage "the diverse and conflicting views" of its support partners if the arrangement was to be stable.
Mr Key's formal inking of the deals paves the way for him to announce his Cabinet tomorrow afternoon and for him and his ministers to be sworn in on Wednesday.
That will allow Mr Key to fly out on Thursday to the Apec summit in Peru as New Zealand's new Prime Minister.
Parliament will be recalled to sit on December 8 for two weeks.
Because the three support parties will hold positions outside Cabinet, they are not part of what will formally be a National minority government and will be free to criticise the government in areas outside their ministerial responsibility.
