Raised in Vermont, Johnson got his start in television commercials. He appeared in one of the very first ads for L.A. Gear. He appeared as a bit character in various television programs before landing the role of Lem on The Shield
. He originally auditioned for the role of ill-fated Terry Crowley, which ultimately went to Reed Diamond. Fortunately for Johnson, series creator Shawn Ryan was so impressed with his audition that the role of Lem was created especially for him.