But his defence team claimed Millane died after encouraging him to choke her during consensual sex, then he disposed of the body and cleaned up the death scene because he was in a panic.
After the three-week trial at Auckland High Cout, they deliberated for about five hours on Friday afternoon before returning the guilty verdict.
Following the jury's verdict, Ms Millane's father David said: "The verdict of murder today will be welcomed by every member of the Millane family and friends of Grace".
The killing shocked New Zealand - which is usually regarded as a safe place to travel - prompting nationwide vigils and a heartfelt apology to Millane's family by Prime Minister Jacinda Ardern.
The killer argued in his defense that she was killed during rough sex, but a forensic pathologist said it would have taken up to 10 minutes for him to strangle her to death.
"Grace was our sunshine and she will be missed forever", the paper quoted her father as saying outside the court.
"In other words, (he) must have appreciated Miss Millane's death was a likely effect. but was willing to run that risk".
"It is natural for you to have sympathy for the Millane family and for Grace, who was here on what should have been a happy and exciting adventure", Auckland High Court Judge Simon Moore told jurors in his summing up, news organisation Stuff reported.
Prosecutor Brian Dickey said that at some point, Millane would have lost consciousness, meaning the man would have needed to keep strangling her after she went lifeless under his grip, news organization RNZ reported.
"It will not reduce the pain and suffering we have had to endure", he said of the verdict.
The killer "eroticised the death of British backpacker Grace Millane" because of his "morbid sexual interest".
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The jury's decision was unanimous.
Millane's parents left the court on the 12th day of the trial as they had entered on the 1st - holding hands and in tears.
The 27-year-old defendant, who can not be named for legal reasons, is expected to be sentenced on 21 February over the death.
The killer had "freaked out", his chief of defence Ian Brookie said, before then lying to police and trying cover it up.
Earlier, the court heard he stuffed Ms Millane's body into a suitcase, which was found buried in bushland outside Auckland.
The murderer, who still can not be named, walks into the dock during his High Court trial.
At 4.10pm the jury asked a question - wanting to clarify what the wording meant in part of the question trail they are considering in their deliberations.
They asked: "Does "when he applied pressure" refer to when he started to apply pressure at the beginning or can it refer to any time when he applied pressure?"
Justice Moore said: "It is not limited to the beginning".
Justice Moore told the jurors it had been a particularly hard trial and discharged them from further jury service for seven years. "I add my thanks to that expressed by David and Gillian Millane, for the way police have conducted this inquiry and supported Grace's family throughout".