When did John Key become prime minister?

Key was made leader of the National Party within the caucus in 2006 after Don Brash's resignation.

Following the 2008 general election which resulted in National's pluarality of votes, National formed a coalition government with ACT and the Maori Party, gaining a majority in the House, and placing a majority of support for Key as leader.

This arrangement was recognised by the Governor-General, officially making John Key the Prime Minister.