Alston was a member of U.S. Soccer’s Residency Program and the Under-17 National Team from September 2003 through his enrollment at Indiana in January 2006. He appeared in 76 matches while at the Residency Program, starting 51 and recording one goal and three assists. He earned 17 caps as a member of the U-17 National Team, including three starts at the 2005 FIFA U-17 World Championship in Peru. He also represented the U.S. at the Pan American Games in Brazil in the summer of 2007.

Alston played college soccer for the Indiana University Hoosiers, where he was a two-time All-Big Ten selection in 2007 and 2008, and a 2008 NSCAA All-Great Lakes region selection.

Alston was drafted in the first round (10th overall) of the 2009 MLS SuperDraft by New England Revolution after signing a Generation Adidas contract with Major League Soccer. He made his professional debut on March 21, 2009, in New England’s first game of the 2009 MLS season against the San Jose Earthquakes. Alston was selected to his first Major League Soccer All-Star Game during the 2010 season and was in the starting lineup for the match in Houston against Manchester United.

On December 22, 2009, Alston received his first call up to train with the senior US national team in preparation for a January friendly match against Honduras, however, he suffered a hamstring injury in training and was unable to participate. Alston was called up for the following friendly against El Salvador, but again had to pull out after a recurrence of the hamstring injury. He has yet to make his senior national team debut.

On April 8, 2013 it was announced that Alston had taken an indefinite leave of absence from the team to undergo treatment for chronic myelogenous leukemia, a rare but treatable form of leukemia. In July 2013, Alston returned to soccer and was removed from the disabled list.

Alston scored his first Major League Soccer goal on April 12, 2014 in 2–0 victory against Houston Dynamo.

On June 8, 2018, Alston joined United Soccer League side Orange County SC.