Sonja's Korean mother met her American father of African, European and Native American descent while he was serving in the U.S. army in Korea. Her older brother, Roger, was born in Korea. Sonja grew up in Newport News, Virginia and attended Warwick High School where she was a cheerleader and homecoming queen.
Sonja uses her mother's maiden name of "Sohn" as her professional name. Sonja describes her mother as a very hard worker who worked for the National Parks Service. Her mother died from cancer in 1998. Sonja moved to New York in 1984 where she eventually married and had two daughters, Sakira and Sophia. Her first marriage ended in divorce.
She became interested in writing poetry after taking a creative writing She began doing performance poetry in which lyrics or poems are spoken aloud to an audience and this led her to acting. She also taught a poetry at Riker's Island prison. Her film debut was in 1996 in Work and she attended acting workshops. Sonja's next film and big break was 1998's Slam, which was awarded the Sundance Grand Jury Award for best feature film.
She appeared in other films before getting the role of Detective Shakima "Kima" Greggs in The Wire in 2002. She has also had a recurring role on Cold Case. Sonja lives in Maryland with her Australian husband, Adam Plack, a composer and musician.