Stefani said Shelton managed to keep his proposal plans secret and caught her off guard.
"It was actually pretty magical," she said. "Wasn't expecting it at the moment that it happened, at all. Didn't have any idea, nobody knew. I think he had told my dad on my birthday about two weeks earlier."
Stefani and Shelton started dating in 2015 following their respective splits from Gavin Rossdale and Miranda Lambert. On The Tonight Show, Stefani said she was beginning to have concerns about her future with Shelton when he finally proposed.
"I got to tell you, I was sort of like, 'What's happening with us?'" she said. "Like, it was kind of in my mind, like 'We've been together for a long time now. What's going on?' I was in that place in my head."
Shelton proposed during a trip with family to their new home in Oklahoma. Stefani said the trip nearly didn't happen because of the COVID-19 pandemic.
"We were this close to canceling the trip. We ended up making it all happen, and we were all here at the ranch and basically it happened," she said. "It was pretty magical."
Shelton kept the engagement ring in his truck and eventually hid it in a cabinet at the ranch. Shelton proposed after asking Stefani to get a fire starter from the cabinet, where she found the ring.
"I open the cabinet and there's this ring there. I'm like, 'Are you serious?' We both start bawling," Stefani said.
Stefani previously discussed her engagement during an interview on The Kelly Clarkson Show in December.
On The Tonight Show, Stefani also performed her new single "Let Me Reintroduce Myself." The song is her first non-festive solo single since "Misery," released in 2016, and will appear on her forthcoming fifth album.