In 1978, even though he had owned a skateboard for only just over a year, Mullen placed fifth in the Boy's Freestyle category at the US Open Championships at Kona Skatepark in Jacksonville, Florida. Skateboard manufacturer Bruce Walker saw his performance and sponsored Mullen through Walker Skateboards from 1978 to 1980. Mullen's biggest influence in skateboarding at the time was a Walker professional skateboarder, Jim McCall, who was coached in his early years by Walker (Walker also coached a young Kelly Slater). Mullen was also influenced in a positive manner by professional skateboarders from Florida including Ed Womble, George McClellan, Clyde Rodgers, Tim Scroggs, and Kelly Lynn.