In 2004, Cook was named to the University of Illinois All-Century Team.
Cook played high school basketball at Lincoln Community High School in central Illinois where he led the Railsplitters to the quarterfinals of the Illinois High School Association class AA state boys basketball tournament. He was named to the 1998 State Farm Holiday Classic all-tournament team and was a 1999 McDonald's All-American. He was also named the 1999 Illinois Mr. Basketball after averaging 21.7 points, 10.1 rebounds, 3.2 blocks, 2.0 steals and 1.8 assists per game as a senior.
Cook played 132 games in four years for the University of Illinois, most of them under coach Bill Self, and led the Illini in rebounding in each season. Cook was a versatile scorer from both inside and outside the paint, utilizing his height to score in the post and hitting three-point shots when left open. This helped him to earn co-Big Ten Freshman of the Year honors during his freshman year at Illinois.
Cook helped lead the Illini to a number one seed in the 2001 NCAA Tournament, and the Illini cruised to the Elite 8, where they were upset in a hard fought and controversial game by Cook's future teammate Luke Walton and the Arizona Wildcats.
As a senior in the 2002–03 season, Cook led the Fighting Illini in scoring with 20.0 points per game, and received the Chicago Tribune Silver Basketball as the Most Valuable Player of the Big Ten Conference. That same season, Cook was named second team All-American by The Sporting News, and third team All-American by the Associated Press, the National Association of Basketball Coaches, and The Basketball Times, as well as Big Ten Player of the Year and first team All-Big Ten by both the coaches and the media. Cook left Illinois as the school's third all-time leading scorer with 1,748 total points, at an average of 13.2 points per game, behind Deon Thomas and Kiwane Garris.
Cook was selected with the 24th overall pick in the 2003 NBA draft by the Los Angeles Lakers. He played sparingly in his rookie campaign as a backup to superstar center Shaquille O'Neal, and was mostly an interior player, notching only five three-point attempts out of 141 total field goal attempts. As his professional career evolved, however, Cook once again became a player who could play beyond the perimeter, after O'Neal moved on to the Miami Heat and Rudy Tomjanovich took over for Phil Jackson as Lakers' head coach. He was mainly used as a three-point specialist; 199 of 422 (or 47.2%) of Cook's field goal attempts were from behind the three-point arc. When Phil Jackson returned to coach the Lakers in 2005–06, Cook started to take less three-pointers again, which resulted in Cook improving his overall field-goal percentage from .417 in 2004–05 to .520 in 2005–06. It also resulted in an improvement in his three-point field-goal percentage, from .392 in 2004–05 to .441 in 2005–06. His improved play kept Cook on the court more as his minutes played per game rose from 15.1 in 2004–05 to 19.4 in 2005–06.
On November 20, 2007, Cook was traded, along with Maurice Evans, to the Orlando Magic in exchange for Trevor Ariza.
On February 19, 2009, Cook was traded to the Houston Rockets in a three-team trade also involving the Magic and the Memphis Grizzlies. On February 20, 2010, he was waived by the Rockets.
On July 9, 2010, Cook signed with the Los Angeles Clippers.
On March 15, 2012, Cook was traded, along with a 2015 second-round pick, to the Washington Wizards in exchange for Nick Young. On September 17, 2012, he re-signed with the Wizards. However, he was later waived by the Wizards on October 28, 2012.
In March 2013, Cook signed with Piratas de Quebradillas for the 2013 BSN season.
On September 30, 2013, Cook signed with the Utah Jazz. However, he was later waived by the Jazz on October 26, 2013.
On September 25, 2014, Cook signed with the Detroit Pistons. However, he was later waived by the Pistons on October 20, 2014. On December 28, 2014, he signed with Al-Riyadi of the Lebanese Basketball League. He left Al-Riyadi in mid-January after appearing in just three games. On February 10, 2015, he signed with Aguada of the Liga Uruguaya de Basketball, in Montevideo, Uruguay. He also managed just three games for Aguada.
On June 15, 2015, Cook signed with Chiba Jets of the Japanese National Basketball League.
Cook is the son of Norman and Joyce Cook, and has two younger sisters, Kristina and Natasha. His father was an All-American with the University of Kansas and played briefly for the Boston Celtics. His uncle, Joe Cook, played basketball for Duke University from 1988–1990.
On July 4, 2009, Cook married long-time girlfriend, Victoria Velasquez.