Johnson is a 1971 graduate of Cardinal Gibbons High School. In 1994 he graduated with an Associate of Arts (Criminal Justice) from. Johnson is also a 1996 graduate of the National Sheriff"s Institute and a 1999 graduate of the Federal Bureau of Investigation National Academy.
Johnson served from 1972 to 1994 in the Anne Arundel County Police Department, rising through the ranks from cadet to commander. He served as sheriff of Anne Arundel County, Maryland, for three terms from 1994 until 2007, winning reelection in 1998 and 2002. In 2006, Johnson unsuccessfully ran for County Executive of Anne Arundel County, losing to John R. Leopold (R).
In 2010 Johnson passed on a rematch against Leopold, even as allegations of misconduct surrounding the incumbent began to surface.
In 2014, Johnson again ran for the county executive seat, running unopposed in the Democratic primary after Joanna Conti, who ran in 2006, declined to challenge him. Johnson lost to Republican State Delegate Steve Schuh in the general election.
