The young woman, from Wickford, Essex, was strangled by the man she had met on dating site Tinder, and with whom she spent several hours drinking in the centre of Auckland on 1 December, 2018.
The pair returned to his apartment and Ms Millane was killed the night before her 22nd birthday.
The jury of seven women and five men returned a unanimous verdict after just five hours of deliberation at Auckland High Court.
Prosecutors successfully argued the murderer, who cannot be named for legal reasons, shoved Ms Millane’s body inside a suitcase before burying her in the Waitakere Ranges outside the city.
She was assumed missing and her father, David Millane, flew to New Zealand as local authorities spent a week searching until her body was found by police.
Grace’s mother Gill broke into silent sobs as the verdict was read out.
The killer – who was described as a “sociopath” during the trial – remained impassive before he was led away to the cells.
He was then brought back to the dock, wearing a dark suit and black open-necked shirt, to be told by Judge Simon Moore that he would be sentenced next year.
Outside court, Ms Millane’s father told reporters: “The verdict of murder today will be welcomed by every member of the Millane family and friends of Grace.
“It will not reduce the pain and the suffering we have had to endure over the past year.
“Grace was taken away from us in the most brutal fashion a year ago and our lives have been ripped apart.
“This will be with us for the rest of our lives. Grace was a beautiful, talented, loving daughter.
“Grace was our sunshine and she will be missed forever.
“She did not deserve to be murdered in such a barbaric way during her gap year.”
Mr Millane acknowledged the efforts of police – and thanked the prosecution team and the media.
“Finally we want to thank the people of New Zealand. I can’t thank you enough for your kindness and for opening your hearts to us,” he said.
“We must return home and try and pick up the pieces of our lives day to day without our beloved Grace,” he added.
The family waved goodbye to their daughter in October last year as she set out on a world tour after graduating from Lincoln University.
After spending time in Peru, she arrived in New Zealand in mid-November, hoping to celebrate her birthday on 2 December.
During the trial, jurors were shown CCTV footage showing the her final hours alive.
She took a picture of a Christmas tree that she sent her parents while she waited to meet her killer for the first time. It was the last time she contacted her family.
CCTV material covering almost her entire date with her killer showed them meeting outside the Sky City casino, drinking at a succession of bars and entering the CityLife hotel where her killer was living.
In a final scene timed at 9.41pm on 1 December, the pair are seen exiting the hotel lift and heading to his apartment.
By the morning, Ms Millane had died. She had been strangled during sex, and the killer had taken a series of intimate photographs of her naked body before watching pornography through the early hours.
Internet searches showed him actively plotting to conceal the murder, while CCTV showed him hiring a car, buying cleaning products, a shovel and a suitcase, which he later used to remove her body from his hotel room and bury in the woods.
At some point during or after this, he arranged another Tinder date for later that day.
Ms Millane’s killer has been remanded in custody until his sentencing on 21 February.
He faces life in prison with a minimum of 10 years.