He appeared for MetroStars, San Jose Earthquakes, Houston Dynamo and Columbus Crew SC in the United States, Eintracht Frankfurt in Germany, and Stabæk in Norway.

Clark spent three years in Europe, playing for Eintracht Frankfurt and Stabæk, before returning to the United States and the Dynamo midway through the 2012 Major League Soccer season.

[3] Clark committed to play college soccer at Furman University,[4] part of a recruiting class ranked fourth in the nation.

Following his sophomore year at Furman, Clark signed a Project-40 contract with Major League Soccer and entered the 2003 MLS SuperDraft, where he was selected second overall by MetroStars.

After appearing 35 times in all competitions with three goals, Clark was named a finalist for the MLS Rookie of the Year Award; he lost out to Damani Ralph of Chicago.

Clark was traded to San Jose Earthquakes on January 14, 2005, in exchange for allocation money and a youth international roster slot.

[17] Playing out of a defensive midfield role, Clark found the back of the net three times on the season: July 2 away to FC Dallas,[18] August 13 against Colorado Rapids,[19] and September 10 against Chivas USA.

Due to San Jose's failure to reach a stadium agreement with AEG, Clark, along with the rest of his Earthquakes teammates, moved to Houston for the 2006 season.

Clark began 2007 by making his continental debut in the 2007 CONCACAF Champions' Cup, appearing in three of Houston's four games against Puntarenas and Pachuca.

Houston was defeated 2–0 after extra time, and with Clark's contract expiring at the end of the season it would mark his final match of his first stint with the club.

Stade Rennais of Ligue 1[27] and newly promoted Serie A club Livorno[28] both reported interest in the player, either to sign him during the summer window or once his contract with MLS expired in January.

Prior to the season finale against VfL Wolfsburg, and despite the injury he had suffered after arriving in Germany, Clark inked a three-year contract with Frankfurt.

He did make his DFB-Pokal debut in late December, playing the full 120 minutes in a 5–3 penalty kick defeat against Alemannia Aachen that eliminated Eintracht from the competition.

Following his release from Eintracht, Clark returned to the United States and signed again with Houston Dynamo, who had retained his Major League Soccer rights when he moved to Germany, on August 2, 2012.

He was officially added to the roster on August 8, after Geoff Cameron finalized a move to Premier League side Stoke City.

He tallied strikes in back-to-back games against Sporting Kansas City and Montreal, sparking the Dynamo to a six-match unbeaten run.

[23] He also scored his first career U.S. Open Cup goal, coming two minutes into stoppage time of a fourth round victory over Austin Aztex.

[48] Clark continued his tear through the second half of the season, tallying match-winning goals against San Jose on July 10 and Vancouver Whitecaps FC on August 29.

On December 5, Houston declined the option on Clark's contract, making him a free agent and ending his eleven-year affiliation with the organization.

Prior to the season, Clark was named as part of the leadership council for Crew SC, joining Federico Higuaín, Hector Jiménez, Jonathan Mensah, Zack Steffen, and Josh Williams.

Clark made his Columbus debut as a late substitute on March 10, replacing Lalas Abubakar six minutes into stoppage time of a 3–2 victory against Montreal Impact.

[65] Clark received his first taste of international soccer with the United States U18 national team, earning six caps and scoring three goals during his two eligible years at that level.

Clark was bumped up to the United States U20 national team at the start of 2002, earning his first cap at that level on January 10 as part of a 1–0 victory over Canada.

[66] His final caps at U20 level came in the United Arab Emirates at the World Youth Championship, where Clark played all five games for the Americans.

He returned to the team after Bob Bradley was named as the new head coach, making his first start for the U.S. in a friendly victory over Denmark on January 20, 2007.

[72] That summer, Clark was named to two American squads: the 2007 CONCACAF Gold Cup in the United States[73] and the 2007 Copa América in Venezuela.

[74] He appeared four times in the Gold Cup, coming off the bench in all three knockout stage matches; in the final against Mexico, he replaced Pablo Mastroeni at halftime as part of the American comeback victory.

Away to Trinidad and Tobago, Clark's 62nd-minute strike was the only score of a 1–0 victory, and guaranteed that the United States could finish no worse than fourth in the Hexagonal.

Against Brazil in the final, he played 88 minutes before being replaced by Conor Casey as the Americans chased the game; they finished as runners-up after a 3–2 defeat to the Seleção.

[81] After appearing in two tune-up matches prior to the tournament, Clark made his World Cup debut on June 12 with a start against England in the group stage.