Herdman defeated his Polish opponent Janusz Walachowski at the City Pavillion in London, reported Daily Mirror.
"I've been training traditional Japanese jujitsu for nearly five years and hold a shodan black belt," Herdman, 28, said.
"It's raw, exciting and unpredictable. I find it more interesting than boxing although I appreciate the beauty and art in boxing. It also made sense to move onto MMA because of my jujitsu training."
Herdman hasn't lost any love for acting, but he compared it to "playing the lottery for a living."
Speaking of acting and fighting, did Herdman ever have any fight with the other Harry Potter kids? "No, we truly all got on pretty well.
"Jamie (Waylett, who played Vincent Crabbe) and I didn't get on in the early days but towards the end he was probably one of my closest friends on set. He's a really funny guy. He's a good rapper too.