[5] During his professional career, he played as a defender for clubs in France, Belgium, England, Italy, the Netherlands, Portugal, Spain, and the United States.

Having been the subject of many transfer rumors regarding clubs throughout Europe, Onyewu finally completed a loan deal with Newcastle United in January 2007, covering the remainder of the 2006–07 season.

Following the arrival of new manager Sam Allardyce, Onyewu fell out of favor, and Newcastle decided not to make the loan deal permanent.

Onyewu dealt with racism during his time in Belgium, even having been punched and shouted at by racist fans while playing for Standard Liège.

"[10] Approximately two weeks later, Onyewu's lawyer announced that he was suing Van Damme in an effort to end on-field racism in European soccer.

[18][19] On November 5, 2010, Onyewu made international headlines when he and teammate Zlatan Ibrahimović engaged in a fight during training,[20] before a league match against Bari.

[27][28] He scored his first goal for Sporting CP against Rio Ave at the Estádio dos Arcos on September 19, a week after making his Sporting CP debut, in a 3–2 win in which Onyewu scored the winning goal from a header after a Diego Capel corner kick found him unmarked in the penalty area.

Despite the sacking of Domingos Paciência and the introduction of Ricardo Sá Pinto as Sporting manager in early 2012, Onyewu remained a first team regular until mid February where he sustained an injury in a league match against Paços de Ferreira.

[31] Onyewu's injury, which turned out to be an internal lateral ligament and an external meniscus fracture to his right knee, kept the American out of action for two months.

The appointment of Ricardo Sá Pinto as Sporting manager led to the arrival of Khalid Boulahrouz and Marcos Rojo, which saw Onyewu loaned out to Spanish La Liga outfit Málaga in August 2012.

[36] A week later, he started and scored his first Málaga goal against CP Cacereño on October 31 in a fourth-round Copa del Rey tie.

[37] Onyewu's goal-scoring form continued in the same competition against SD Eibar on December 18, where his stoppage time goal earned his side a draw.

[38] However, Onyewu's stay with Málaga proved to be unsuccessful as he struggled to establish himself as a first-team regular and only managed to play nine matches all season, contributing two goals.

[39] In October 2013, Onyewu signed with Queens Park Rangers on a free transfer,[40] but he failed to make any appearances and was an unused substitute on six occasions.

[46] Originally brought in to be a veteran presence for a young backline Philadelphia, Onyewu became a regular starter for the Union earning 22 appearances and scoring once.

Onyewu scored his first international goal in the 2005 Gold Cup, nodding home the extra time winner in the semifinal against Honduras, and was later named to the tournament Best XI.

During the 2009 Confederations Cup, Onyewu was paired with Jay DeMerit due to the injury of his normal partner, Carlos Bocanegra, who was unable to play in the group stage.

[55] During the 2010 World Cup, Onyewu started and played the full 90 minutes in the United States's first group match against England.

[59] At 6 ft 4 in (1.93 m) and 210 lb (95 kg), Onyewu is the second-tallest outfield player in United States national team history (after Omar Gonzalez).