The husky kept hiking with her but a few kilometres later she slipped trying to cross a freezing river.
Struggling to stay afloat, Milling knew she'd have to take off her backpack.
"I was really struggling to breathe," she told the Democrat and Chronicle.
"My whole body was numb."
She tried to reach for a branch but couldn't hold on.
Nanook came to the rescue for a second time, jumping in the frigid waters and grabbing her backpack in his mouth while paddling her to dry land.
Milling grabbed her satellite phone and called for help.
Alaska state troopers rescued the pair the following day.
Nanook was soon returned to his owner, Scott Swift, who lives kilometres from the hiking trail.
Milling described Nanook as her "guardian angel", recalling how Nanook even stayed outside her tent when she camped during the night.
"I told him several times that I love him," she said.
"And I'll never forget him."
According to Nanook's owner, his husky has saved at least three other hikers, and said he "wouldn't doubt if there's more out there".
Swift started a Facebook page for the pup to find other people he'd helped. Within a few days, he heard dozens of stories.