Sohn is of East African and Korean heritage. Before she was an actress, Sohn was a slam poet, having written lyrics and co-written the script for the film Slam. Throughout the five seasons of the HBO series The Wire, she held a starring role as Detective Kima Greggs. She has also guest starred on many episodes of Cold Case as Toni Halstead, the love interest of starring character Nick Vera. Sohn won the supporting television actress award at the 2008 Asian Excellence Awards for her character on The Wire.

Having previously been involved in political activism (she campaigned in North Carolina in support of Barack Obama's 2008 presidential bid), Sohn took a break from acting in 2009 in order to concentrate on social issues. She is now the founder and CEO of the Baltimore-based reWIRED for Change, an outreach program intended to communicate with (and ultimately rehabilitate) at-risk young people who have been involved in criminal activity. The program is run out of the University of Maryland School of Social Work and uses episodes of The Wire as a teaching tool, encouraging the participants to examine and query their own lives and past actions. Various other actors and writers involved with The Wire also serve as board members.

Sohn will appear in the upcoming medical drama television series Body of Proof, which will premiere on ABC in Fall 2010.

Sohn is a graduate of Warwick High School where she was elected homecoming queen. She has two daughters and is married to Australian composer Adam Plack.