The 34-year-old finished the gruelling 2.4-mile swim, 112-mile bike ride and 26.2-mile run in nine hours, 15 minutes and 54 seconds.
She was just over a minute clear of Switzerland's Caroline Steffen.
It was Cave's second world title of the year after victory in the Half Ironman World Championship last month.
"I'm kind of speechless," said Cave, who finished third behind fellow Briton Chrissie Wellington and Australian Mirinda Carfrae in 2011.
"I just hope they don't come out with another distance. I'm getting too old for this."
Britain's Rachel Joyce finished 11th in 9:40:16, while double Olympic medallist Rebecca Romero, who took up Ironman after retiring from cycling, came home in 11:39:08.
Cave was always in contention at the front of the field, exiting the swim in fourth and the bike in third, then pulling clear of Australian rival Carfrae on the run before overhauling Steffen.
Australia's Pete Jacobs won the men's event, beating Germany's Andreas Raelert by more than four minutes.

Ian Brown, 44, and Daniel Dunkley, 35, died following incidents in their cells at HMP Woodhill last July.
Their relatives claim Woodhill's governor has not complied fully with Prison Service Instructions (PSIs).
But the governor and the Secretary of State say the judicial review claim is "neither appropriate or necessary".
PSIs cover management of prisoners at risk of harm to self, others and from others, early days in custody and medical emergency response codes.
Heather Williams QC, for the families, said the claim addressed the "exceptionally high" rate of self-inflicted deaths at the Milton Keynes prison.
There were seven self-inflicted deaths last year, five in 2015 and 18 at the prison since May 2013.
"The rate of self-inflicted death at HMP Woodhill is far higher than at any other prison, at a time when the suicide rate in the prison estate as a whole is at a record high," said Ms Williams.
She said the case raised serious ongoing breaches of Article 2 of the Human Rights Act, which protects the right to life, and involved long-term failures to comply with the responsibility placed on the authorities to protect prisoners.
James Strachan QC said the governor was well aware of his obligations to comply with the requirements of the PSIs.
"The defendants do not dispute that the number of deaths at the prison is a legitimate matter of concern.
"However, not only have the governor and the Secretary of State taken significant action in 2016 to improve the situation, but as the taskforce approach shows, this is a continuing high priority."
Lord Justice Irwin and Mr Justice Garnham will give their decision at a later date.

Glyndwr University is hosting the first meeting of the North Wales Cyber Security Cluster with local businesses also attending.
Prof Vic Grout said the aim is to meet monthly to share good practice.
Security agency GCHQ said people and businesses could adopt basic cyber security principals to keep safe.
Britain's Leanda Cave pulled away in the last three miles of the run to win the Ironman World Championship in Hawaii.
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Families of two men who died in prison have brought a High Court case over the "exceptionally" high rate of self-inflicted deaths there.
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Police and experts in cybercrime are meeting in Wrexham to discuss ways to tackle the issue and keep people safe online.