Herdman started acting when he was about 7 years old; his father found him an agent. At age 13, he was cast as Gregory Goyle, a sidekick of villain Draco Malfoy, in Harry Potter and the Philosopher's Stone (2001). Over the next decade, he returned as Goyle in all seven sequels. He had originally been in the running to play Dudley Dursley, Harry Potter's spoiled cousin, but the role went to Harry Melling.
After five years of practising jiujitsu, Herdman began his MMA career on 23 April 2016 in Romford with a decision win over Janusz Walachowski. He said, "I chose MMA because I love the sport. 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. I would like a few amateur fights first to get me started and where it goes from there, who knows?"