Johnson attended Eastern Michigan University on a partial athletic scholarship for American football, but he was expelled after he was caught stealing food from the school cafeteria.

In 1993, Johnson became part of the original cast of the television series Homicide: Life on the Street playing Detective Meldrick Lewis for all seven seasons and the reunion movie, as well as directing several episodes.

Johnson regularly improvised during filming and made up his own jokes and dialogue; writer and producer James Yoshimura called Clark the "king of the ad lib".

The two detectives became the central figures in a plot line surrounding a Baltimore drug lord whose financial resources and front as a devoted community servant make it nearly impossible for the police department to charge him.

[6] In 2013, Johnson made a brief cameo as Lewis in the Law & Order: Special Victims Unit episode "Wonderland Story" when the squad are at a retirement party for John Munch (Richard Belzer).

[15] In 2013, Johnson starred as Sen. Robert Bettencourt (R-PA) in Amazon's Alpha House, a political comedy written by Doonesbury creator Garry Trudeau.

The series stars fellow The Wire cast members Pablo Schreiber and Reg E. Cathey and focuses on a retired heavyweight boxing champion.