Clarkson, 37, and Trainor, 25, recalled on Thursday's episode of The Kelly Clarkson Show how they jumped in a photo with Obama during the former first lady's visit to Today.
Obama was celebrating International Day of the Girl in October 2018 when she posed for a selfie with model Karlie Kloss, actresses Zendaya and Frieda Pinto, and activist Monica Nyiraguhabwa. Clarkson and Trainor, who were also on stage, leapt in the background.
"The best part is like Where's Waldo, find the two classiest people here. That'd be us in the back," Clarkson said. "Everyone else was meant to be there and we were like, 'Whatever, we're crashing.'"
"Yeah, I yelled at you. I was like, 'Get in! It's our last chance,'" Trainor recalled. "She was about to leave, so that was it. You're welcome."
"I just yelled at [Clarkson], like, 'Photobomb this! Get in every picture you can 'cause it's like the coolest day of our lives,'" she said.
Trainor said she looked at Obama and screamed "This is the greatest day of my life!" before hugging the former first lady.
"I think she liked it. She hugged me," she said.
Trainor returned to Today this month to perform "With You," her new song with Kaskade. She will release her third studio album, Treat Myself, in 2020.