Born in Vicksburg, Mississippi, Johnson attended the Vicksburg Public Schools, graduating from Temple High School. He received a bachelor's degree in political science from Tennessee State University and a master's degree in political science from the University of Cincinnati.
His professional career includes the founding of the Mississippi Institute of Small Towns, a non-profit agency developed to assist small economically depressed towns with minority leadership with housing, community development and infrastructure needs.
In 1993, Johnson ran unsuccessfully for mayor of Jackson, placing third in the Democratic primary behind two white candidates, incumbent mayor J. Kane Ditto and former mayor Dale Danks. But in 1997, Johnson defeated Ditto in the Democratic primary, and later defeated Republican opponent Charlotte Reeves in the general election, becoming Jackson's first African-American mayor.
Supporters credited Johnson with overseeing a dramatic renaissance in the city, leading the charge for several projects to help revive a decaying downtown area, including the revitalization of the Farish Street entertainment district and the strong passage of a controversial bond issue to build a convention center. Critics charged that he was an unresponsive leader who was not appropriately concerned with the city's large crime problem, neglected the maintenance of streets, and did little to curb the exodus of Jackson's upper- and middle-class residents to places outside the city limits.
In 2005, Johnson was challenged in the Democratic primary by Frank Melton, a controversial TV station manager who promised to solve the city's crime problem in 90 days while giving few specific crime plans, making crime the central focus of the election. Critics charged that Johnson said crime was only a "perception," although his perception complaints were, in fact, about the "perception of hopelessness" that he argued that crime sensationalism by the media promoted in the city. Even though Johnson did not actually say that crime was only a "perception," the phrase "perception of crime" was used widely by opponents and critics of the incumbent mayor. Melton defeated the incumbent by 63 percent of the vote in the Democratic Primary. Melton went on to easily defeat Republican challenger Rick Whitlow in the general election and succeeded Johnson as mayor of Jackson.