[5] At a young age Simmie's parents divorced and he was sent to live on his aunt and uncle's sharecropper farm with his eight cousins in Leroy, Alabama.

[1][4] Knox began his career teaching at the Bowie State College, Maryland and the Duke Ellington School of the Arts, Washington D.C.

[6] He subsequently painted notable figures such as Muhammad Ali, and Supreme Court Justices Thurgood Marshall and Ruth Bader Ginsburg, before coming to the attention of the U.S. Senate and the White House.

[6][9] The paintings of Bill and Hillary Clinton took two years to complete, finished in 2002[1] and unveiled in June 2004, hanging in the White House's East Wing.

[1] Knox has been described as "the unofficial portraitist for trailblazing African Americans",[8] adding paintings to his portfolio of U.S. Attorney General Eric Holder, Governor Andrew Cuomo and a sculpture of mayor of Baltimore, Clarence Burns.

[8] Knox produced portraits of Joseph A. Johnson Jr., James Lawson, Walter R. Murray Jr. and Perry Wallace, four African-American alumni of Vanderbilt University, in 2018.