Doug's interest in Mixed Martial Arts started with the early UFC tournaments. Doug and his friends would gather and watch them all. Being the largest among his friends, The Rhino was no stranger to fighting as he was often called upon to stick up for his friends. But it wasn't until a challenge made by his father that Doug started to pursue fighting in the cage. In 2003, he was at a WEC event with his dad. He pointed to a fighter in the cage named Anthony Arria and stated that he could "kick that guy's ass!". Doug's father laughed and told him to go do it.

Three months later, Doug "The Rhino" Marhsall won his first professional fight. It took only 32 seconds! The sensation of putting it all out there in the cage had Doug hooked and he set forward on his MMA career. After winning his second fight in the WEC, Doug was given the opportunity to prove to his father that he meant what he said. He was offered a title fight against Anthony Arria and won via armbar in only 22 seconds.

These early successes drove Marshall to train even harder. After adapting a true MMA training regimen, he dropped down from 250 pounds to fight in the Light Heavyweight division. On August 18th, 2006 The Rhino faced Lodune Sincaid for the WEC Light Heavyweight Championship. Doug knocked the champion out to become the new title holder. Since then, his reputation has continued to grow and he continues to make a name for himself in the WEC.

Doug does his conditioning and Muay Thai training at the Valley Fight Club in Hanford, California and trains his jiu jitsu ground game under Matt Smith at Pacific Martial Arts, a Dave Camarillo Academy. He brings it all together under MMA coach Mike Popp.