Williamson earned a gold medal at the 2007 World Championships in Osaka, Japan, running the third leg of the 4x400-meter relay as the United States finished first in 2:55.56.
In 2008, Williamson finished in sixth place in the 400 meters at the U.S. Olympic Team Trials, earning a spot on the USA Olympic team as a member of the six-man 4x400-meter relay pool.
Since signing a professional contract in the summer of 2005, Williamson finished second to teammate Jeremy Wariner at the U.S. Championships and took seventh at the World Championships in Helsinki, Finland. He is one of only two runners to defeat Wariner in the 400 meters over the past five years, accomplishing the feat on three occasions.
Williamson, a San Antonio native, is one of the most decorated athletes in the storied history of Baylor Track & Field, capturing four NCAA titles, 13 All-America honors and 13 Big 12 Conference championships. He was an All-American all four years outdoors in both the 400 meters and as the anchor of the 4x400-meter relay, which captured the outdoor title in 2004. He was a two-time individual All-American and three-time relay All-American indoors, anchoring the Bears to NCAA 4x400-meter relay titles in 2002 and 2004. The Bears' 2004 squad also broke the NCAA record with a time of 3:03.96.
In 2005, Williamson completed an unprecedented sweep by claiming his fourth straight Big 12 outdoor 400-meter title in addition to anchoring his fourth straight relay crown. He also won the Big 12 indoor title to go along with his 2002 title and anchored the indoor relay to the championship, adding to the titles he and the Bears also won in 2002 and 2004.
Williamson was named the 2004 Big 12 Conference Outdoor Performer of the Year and was named the conference athlete of the week multiple times during his career.