He came out on top of a rivalry with Jeremy Wariner in 2008 by winning in the 2008 Olympic final in a personal best time, and by a record margin. He also broke the Olympic record in the relay with the American team, recording the second fastest time ever. Merritt established himself as the World Champion with a win at the 2009 World Championships in Athletics in the 400 m and the 4×400 m relay. In October 2010, he was suspended for 21 months for failing a drug test.
Prior to the 400 m final at the 2007 World Championships in Athletics in Osaka, Merritt stated his intent to beat all-comers. He achieved his first sub-44 second run, finishing in 43.96, and beat 2000 Olympic champion Angelo Taylor to the line. However, this was not enough to beat the reigning World and Olympic champion Jeremy Wariner, who was half a second ahead. Nevertheless, the silver medal was Merritt's first at a global championships over the 400 m. He again formed part of the United States' 4×400 meter relay team and, with fellow individual medallists Wariner and Taylor among the team, the American's eased to victory some three and a half seconds ahead of the Bahamians. With Wariner absent from the field, Merritt won the gold medal at the 2007 IAAF World Athletics Final.
Merritt's 2008 season was distinguished by a considerable rivalry with Wariner, who had won the 400 m at every major global championship since 2004. The 2008 IAAF Golden League provided the venue for many of their duels. He scored his first major win over Wariner in a close affair at the Internationales Stadionfest in Berlin. He confirmed his Olympic place a month later by winning at the 2008 United States Olympic Trials, again defeating the reigning Olympic champion Wariner. Later in July at the Golden Gala meeting, Wariner responded by edging a win in the 400 m by just 0.01 seconds. At the Meeting Gaz de France in Paris, the last Golden League competition before the Olympics, Wariner seemed to have the momentum behind him after a win in 43.86 seconds.
Merritt winning 2008 Olympic gold, a second ahead of Jeremy Wariner
Merritt won the 400 m at the 2008 Summer Olympics in Beijing. A close race between Merritt and Wariner was expected, though it ultimately ended in a rout. The 0.99 second margin between Merritt's first-place finish and Wariner's second-place finish was the largest in an Olympic 400 m final. His time of 43.75, a new personal best, made him the fifth fastest 400 m runner on the all-time lists, still two places behind Wariner, who is third on the all-time list of fastest runners. He teamed up with Wariner, Angelo Taylor and 400 m bronze medallist David Neville for the men's 4×400 m relay. The team defeated the Olympic record mark with had stood since the 1992 Barcelona Olympics by running a time of 2:55.39, the second fastest in the history of the event.
Weeks after the Olympics, he lost to Wariner by a large margin at the Weltklasse Zürich, although Wariner's winning time of 43.82 seconds was still slower than Merritt's Olympic winning run. Merritt secured his fourth win over Wariner that season at the 2008 IAAF World Athletics Final. Although the two had both beaten each other that season, Merritt had won all the most important races, ending the season as the Olympic and American champion over 400 m as well as taking home the World Athletics Final payday. He opted to miss out on the 2009 indoor season to focus on improving his running and technique.
Merritt en route to becoming 400 m world champion in 2009
With Wariner already qualified for the World Championships as the defending champion, Merritt won the 400 m at the 2009 US Championships somewhat uncontested, equalling his own world leading time of 44.50 seconds. At the 2009 World Championships in Athletics, in Berlin, he went on to win the 400 m in a world-leading time of 44.06 seconds, once again beating Wariner.
On 22 April 2010 it was revealed he had failed three drugs tests and as a result he accepted a provisional suspension. He claimed that the failed drug tests resulted from his use of an over-the-counter penis enlargement product, ExtenZe. Merritt stated that he had not read the small print to check the ingredients of the product, which contains the banned steroidDehydroepiandrosterone. He accepted a two-year ban for the infraction and stated that he had made a "foolish, immature and egotistical mistake...Any penalty I may receive for my action will not overshadow the embarrassment and humiliation I feel".
After his two-year ban was reduced to 21 months, LaShawn Merritt finished second at the Stockholm meeting of the Diamond League series with a time of 44.74. He received a berth to the 2011 World Championships in Daegu, South Korea due to his being the 2009 World Champion for the 400 meters. At the 2011 World Championships, he set a world leading time of 44.35. He eventually won the silver medal behind teenager Kirani James of Grenada, having led most of the race, but went on to run the final leg of the United States' gold medal winning 4x400m relay team having been in third place coming out of the final bend.