Onyewu's parents moved to the United States from Nigeria to attend Howard University in Washington, D.C. Onyewu has two brothers, Uche and Nonye, and two sisters, Chi-Chi and Ogechi. Growing up in Silver Spring and later Olney, Maryland, Onyewu attended St. Andrew Apostle School and Sherwood High School, where he played soccer for two years. He then enrolled in the U.S. residency program in Bradenton, Florida, before returning to Sherwood to graduate. He also holds Belgian citizenship.

At 6 ft 4 in (1.93 m) and 210 lb (95 kg), Onyewu is the tallest outfield player in U.S. team history (two goalkeepers have been taller).