Merritt came to prominence as a junior athlete at the 2004 World Junior Championships in Athletics. He took the gold medal in the 400 meters race and set two junior world records as part of the American 4×100 and 4×400 meter relay teams. He took part in the 2005 World Championships in Athletics, his first major senior championship, and acted as the relay substitute for the men's 4×400  m. He helped the team win their heat and was substituted for Jeremy Wariner for the final, where the American team won the gold medal.