Cyrus announced the performance on Thursday night's episode of The Tonight Show Starring Jimmy Fallon, where she spoke about the song's recording process as well as working with Lennon and Ono's only son.
"So, we went to Electric Lady (Studios) with Sean Ono Lennon and we got to cut 'War is Over,'" Cyrus told Fallon. "And so we're also doing it on SNL. We want to do a Christmas song, and I don’t think anything could speak louder than what’s going on right now than about 'War is Over.'"
As promised, Sean joined Cyrus and Ronson on the SNL stage.
Cyrus and Ronson also performed the previously released "Nothing Breaks Like a Heart," the lead single off Ronson's upcoming album, Late Night Feelings. The project will mark the DJ/producer's first full-length release since 2015's Upton Special.
During a recent interview with the BBC, Ronson discussed the forthcoming album and how he and Cyrus connected for "Nothing Breaks Like a Heart."
"I heard the drawl in her voice and the twang when I saw her live on SNL. My jaw dropped and I became super-fixated about being able to work with her," he explained. "But we weren't trying to make a Western song. I think with Miley, just the way she wrote the verses and the way the melody came out, came naturally."