He was sworn in at a ceremony at Government House this afternoon.
His wife Mary and their six children were in attendance, and an audience of around 80 people cheered loudly as Governor General Dame Patsy Reddy announced that Mr English was officially New Zealand's new leader.
Mr English's new ministerial portfolio was revealed at the ceremony - Minister for National Security and Intelligence.
Deputy Prime Minister Paula Bennett was also sworn in, after which Dame Patsy said she wanted to go "off script" and congratulate Ms Bennett, again to loud applause.
In a short statement, Mr English said it was a privilege to be appointed to the country's top job.
Addressing Dame Patsy, he promised to "take seriously the role you have conferred on us".
In his first speech as National's leader today, the incoming Prime Minister said he would vote differently on gay marriage if it came up to the vote now.
Mr English - an "active Catholic" - opposed the gay marriage bill as well as conscience votes on abortion and euthanasia in the past.
While his Catholic faith was a strong factor in his life and had instilled the values he lived by, Mr English said he would vote differently on gay marriage now. He did not intend to run the Government based on Catholic beliefs.
"I don't think gay marriage is an attack on anyone else's marriage."
He also proclaimed himself a "supporter of the monarchy"
"But it would be hard to beat [John Key's] enthusiasm."
Mr Key, meanwhile, today said he "felt great" about standing down.
In his last comments as Prime Minister, Mr Key said: "It's a great day for a new Prime Minister.
"I'll be looking forward to signing off to my caucus colleagues as the leader, after 10 years at the top leading the National Party and eight years the country.
"But I know I'm passing over to tremendously good hands in the form of Bill English and Paula Bennett. They are going to steer the country extremely well."
