The historic coupling took place at the Austin City Limits Music Festival in Texas on Sunday, when Mumford & Co. headlined the final day of the annual event.
Ulrich joined the band onstage at the climax of a 16-song set. "We'd like to invite a friend of ours from a little band called Metallica," bellowed band frontman Marcus Mumford, before introducing Ulrich to the audience.
It is not known if the move is to be made permanent.
Late last week it was confirmed that Metallica would headline five US festivals in 2020, playing two headline sets at each event.
“I really don't think that much about the setlist that much in advance," says Ulrich. "We like to go deep and dust off things that we haven't played for a while.
"It’ll be fun, it’ll be challenging, but we thrive on this shit – and any time we get a chance to do anything outside of the norm, Metallica are four very happy guys.
“I don't want to freak anybody out, but it may not come together until about 24 hours before the first show, but it will definitely come together.”