Jurors in Auckland, New Zealand, unanimously agreed that the man, who cannot be named, was guilty of strangling the 21-year-old.
One woman who walked past with her seven-year-old son said it was "embarrassing" for Auckland.
A 26-year-old man has been charged with the murder of Grace, 22.
New Zealand police are searching for a 22-year-old British backpacker, last seen in Auckland. A CCTV image shows Grace Millane in the Sky City precinct downtown. Millane's family reported her missing when she stopped responding to their messages. She had been staying at Auckland's Base Hostel and was travelling through New Zealand on her own.
The Duke and Duchess of Sussex are nearing the end of their sixteen-day tour or Australia, Fiji, Tonga and New Zealand. In Auckland, they wanged wellies, spoke at a charity event and were hosted at a reception held by the prime Minister, Jacinda Ardern.
So long, 2017.