"I didn’t see you as a Belieber," he said.
"It was great. I went with some friends," Hamm said. "I was like, 'I don’t know, he's got a big album out, couple cool songs.' Like, 'why not?' I met him once doing a digital short on 'SNL' or something."
"Here's what I wanted to hear," Hamm continued. "I wanted to hear 15,000 teenage kids lose their minds. Mission accomplished. It was amazing."
Hamm did notoriously slam Beiber in an interview with Men's Fitness, calling the pop star a "sh*thead" who needs his mommy and daddy to give him some basic home training.
"You see people in the world and you're like, 'Do you know how a washing machine works? Do you know how to wash a dish?'" Hamm said in the 2014 interview. "Life skills are something we're missing ... just shit you needed to learn in life."
Hamm told Fallon he also caught a Beyoncé concert in Atlanta, as well as what turned out to be Prince's last-ever show.
Hamm said the Prince concert was a "jam packed house."
"90 minutes end to end. Never stopped playing. It was amazing, like a religious experience," Hamm said.