A week before then she was told she would pair up with Edward.
So they had precious little time to prepare, but it didn't show.
They finished second in their semifinal behind the South African combination, but in the final it all came together, with a world-best time of 6min 48.560s, heading home Canada's Lindsay Jennerich and Patricia Obeee by a touch under two seconds.
They eclipsed the time set by Edward and Louise Ayling in 2012.
Arms were flung in the air, at least partly at the unexpectedness of what the young women had achieved.
It proved to be a perfect mesh of two talented rowers.
For Edward it was a stellar year, with wins in eight of her nine races in either boat from the second World Cup on.
"I actually couldn't believe it crossing the line," Edward said at the time. "It has been a dream."
As for MacKenzie, who moved from Blenheim to Cambridge three years ago, she has her own website dedicated to healthy eating - she's gluten free - along with Zealand squash representative Megan Craig, called www.megsandsoph.com.
When the world crown was won a sign appeared on the side of State Highway 63 in Marlborough - Sophie MacKenzie Drive.
It sits in the kitchen back home.
The Rio Olympic qualification will take place at this year's world championships in Aiguebelette. Edward and MacKenzie look well set to press on to bigger things - with perhaps another sign in the making.