Roberts and her children, twins Phinnaeus and Hazel and son Henry, were attending Swift's 1989 tour stop in Santa Clara, Calif., when Roberts and Baez joined Swift onstage during her song "Style."
"Taylor had said before the show ... 'Would you like to come on stage?' I was like, 'Oh, I don't know.' I get very nervous," Roberts said.
"And at the time, I think Phin and Hazel were 10 and Henry was 8, and they were like, 'Oh, Mom, you should do it. You should do it.' And Henry goes, 'Don't do it unless you feel comfortable,'" she added.
Roberts asked for a Taylor Swift t-shirt to wear before agreeing to come onstage.
"The whole thing was pretty surreal, and the Joan Baez part of it made it even more surreal," she said.
Swift concluded the first leg of her Eras tour in November and was named Time magazine's Person of the Year on Wednesday.
Leave the World Behind premieres Friday on Netflix.
On The Tonight Show, Roberts also played a game of Box of Lies with host Jimmy Fallon.