He won two Olympic gold medals, the first in the 400 m in a personal best 44.00 seconds and the second as the third leg of the U.S. 4 × 400 m relay team. Following his Olympic successes, he turned professional, forgoing the rest of his collegiate eligibility, though he remained at his parents' house and continued to be coached by Clyde Hart. The next year, Wariner won the 400 m at the 2005 USATF championship with a time of 44.20 s. At the Helsinki World Championships on a cold and rainy day he won the 400 m in 43.93 seconds. He would then anchor the American team in the 4 × 400 m relay for the gold medal. Early in 2006, Wariner competed in the 200 m lowering his personal best to 20.19 s. Later that year he would set a new personal best of 43.62 seconds at 400 m at the Golden Gala Meet in Rome. Together with Asafa Powell (100 m) and Sanya Richards (women's 400 m) he won his sixth out of six Golden League events (400 m) in the same season, which earned him a total of $250,000. In 2007 he filled a summer with dominating 400 m performances culminating with the Osaka World Championships where on August 31, 2007 he would win the 400 m in 43.45 s thereby improving his personal best to become the then-third-fastest of all time (only Michael Johnson and Butch Reynolds had run faster). In 2008, Wariner left long-time coach Clyde Hart in favor of working with Baylor assistant coach Michael Ford. This was an unexpected move as Wariner had much success under Hart, who had also coached his agent Michael Johnson. Wariner stated that he needed a change as Hart was nearing retirement, although the coach said that the split was due to a pay dispute. By the time of the 2008 USATF championship Wariner and Johnson had to face many probing questions regarding the reasons for such a change in an Olympic year. He finished second to LaShawn Merritt in the 400 m final of the USATF championship and secured his place on the U.S. Olympic team in the 400 m event and the 4×400 m relay team. In the 2008 Olympics, Wariner qualified for the final with a time of 44.12 s, in a run which he slowed down considerably in the final fifty meters. This led to much anticipation that he could beat Michael Johnson's world record in the final, but instead he took the silver, losing to LaShawn Merritt by nearly a second.