Clark grew up in Plainfield, New Jersey and attended Rahway High School in Rahway, New Jersey. There, he scored 1,245 career points. As a senior, he averaged 25.2 points, 13.2 rebounds and 5 assists per game. He was a 2006 McDonald's All-American, as well as a fourth-team Parade All-American.
Considered a five-star recruit by Rivals.com, Clark was listed as the No. 8 small forward and the No. 22 player in the nation in 2006.
As a freshman for the Louisville Cardinals, along with Derrick Caracter, Edgar Sosa, and Jerry Smith, the young group struggled early in the 2006–07 season. The team went on to win 8 out of its last 10 games, earning a bid to the NCAA Tournament.
In the first game of the 2007 NCAA Tournament, he scored 12 points, 4 rebounds and 4 steals against Stanford.
Clark decided not to put his name in the 2008 NBA Draft and to return to the University of Louisville for the 2008–2009 season.
In the 2008–2009 season, Clark, along with future NBA player Terrence Williams, led Louisville to the Big East Title, as well as a #1 ranking, the first ever in the University of Louisville's history.
Early in his junior season, Clark announced that he would forgo his senior season to enter the 2009 NBA draft. On April 4, 2009, agent Dan Fegan told ESPN that he had signed Clark as a client shortly after the end of Louisville's 2008–09 season, which under NCAA rules ended Clark's college eligibility.
Clark was drafted 14th overall by the Phoenix Suns in the 2009 NBA draft. He made his first career three pointer on January 28, 2010. Clark was sent to the Iowa Energy of the NBA Development League on March 15, 2010, after only averaging 2.7 points, 1.1 rebounds, and 7.6 minutes in 45 games.
On December 18, 2010, Clark was traded to the Orlando Magic along with Jason Richardson and Hedo Türkoğlu for Vince Carter, Marcin Gortat, Mickaël Piétrus, a 2011 first-round draft pick and $3 million cash. On April 16, 2012, he recorded his first double-double with 14 points and 11 rebounds.
In August 2011 during the 2011 NBA lockout, Clark signed a one-year contract with Zhejiang Lions in China. However, the next month, he asked to leave the team for family reasons after not appearing in a game for the club. He re-signed with Orlando in December.
On August 10, 2012, Clark was traded to the Los Angeles Lakers as part of a four-team trade that sent Dwight Howard to the Lakers. The Denver Nuggets acquired Andre Iguodala, the Philadelphia 76ers received Andrew Bynum and Jason Richardson, and the Orlando Magic got Arron Afflalo, Al Harrington, Nikola Vučević, Maurice Harkless, Josh McRoberts, Christian Eyenga, and one protected future first-round pick from each of the other three teams. The Lakers also acquired Chris Duhon from the Magic in the trade. On January 9, 2013, Clark had a breakthrough night, playing 27 minutes and setting personal bests with 22 points and 13 rebounds against the San Antonio Spurs. He had been playing sparingly, but he received extended playing time after injuries to Lakers big men Dwight Howard, Pau Gasol, and Jordan Hill. Coach Mike D'Antoni called Clark's performance "phenomenal"; it was only the second double-double of Clark's career. Clark had three double-doubles over six games, and he was named a permanent starter even after Gasol returned. After three years of limited opportunities, Clark was averaging nearly 30 minutes a game. However, he appeared tired by March, and D'Antoni replaced Clark with Gasol in the starting lineup.
On July 12, 2013, Clark signed with the Cleveland Cavaliers.
On February 20, 2014, Clark was traded to the Philadelphia 76ers along with Henry Sims and two future second-round picks in exchange for Spencer Hawes. He was waived by the 76ers the next day.
On February 27, 2014, Clark signed a 10-day contract with the New York Knicks. On March 10, 2014, he signed a second 10-day contract with the Knicks. On March 20, 2014, the Knicks did not offer him a rest of season contract.
On September 25, 2014, Clark signed with the Memphis Grizzlies. However, he was later waived by the Grizzlies on October 22, 2014. He was then claimed off waivers by the Houston Rockets on October 24, only to be waived again three days later.
On October 31, 2014, Clark was acquired by the Iowa Energy of the NBA Development League as an affiliate player of the Memphis Grizzlies. However, he was traded to the Rio Grande Valley Vipers the next day.
On December 10, 2014, Clark signed with the Shandong Golden Stars of the Chinese Basketball Association after reaching a buyout with the Rio Grande Valley Vipers. In 19 games for Shandong, he averaged 26.7 points and 10.3 rebounds per game.
On March 27, 2015, Clark signed a 10-day contract with the Brooklyn Nets. On April 6, 2015, he signed a multi-year deal with the Nets. On August 10, 2015, he was waived by the Nets.
On October 30, 2015, Clark was acquired by the Bakersfield Jam of the NBA Development League. On January 16, 2016, he was traded to the Delaware 87ers in exchange for a 2016 second round pick. Three days later, he made his debut for the 87ers in a 127–112 loss to the Westchester Knicks, recording four points, four rebounds, one assist, two steals and two blocks in 21 minutes off the bench. On March 19, he was waived by Delaware.
On May 24, 2016, Clark signed with Henan Shedian Laojiu of the Chinese National Basketball League.
On August 18, 2016, Clark signed with Turkish club Beşiktaş for the 2016–17 season. On July 3, 2017, he re-signed with Beşiktaş for one more season.