Timberlake, 32, said in a British radio interview this week that he and Mumford worked with veteran T-Bone Burnett and folk band The Punch Brothers on the soundtrack for "Inside Llewyn Davis," a film directed by Joel and Ethan Coen and starring Oscar Isaac and Carey Mulligan.
Mulligan married Mumford in April 2012.
"We all kind of worked on the music together and I don't know any other world where we would get the opportunity to collaborate like that, but it was so much fun. So not only will it be a great movie, but the music to it will be fantastic," Timberlake told Britain's Capital FM radio station.
The film, co-starring Timberlake, John Goodman and Garrett Hedlund, follows a singer-songwriter exploring the folk music scene in 1960s New York. It is expected to be released later this year.
The "Sexyback" singer, who first gained fame in the 1990s as lead vocalist of the boy band 'N Sync, stepped away from music five years ago to focus on films such as "The Social Network" and "Friends With Benefits." But last month he released lead single "Suit & Tie" ahead of his album "The 20/20 Experience" out on March 19.
Timberlake is also starring in upcoming crime drama "Runner, Runner," alongside Ben Affleck and Gemma Arterton.
Mumford & Sons won two Grammy Awards earlier in February, including Album of the Year for "Babel."