Clark spokesman Matt Bennett said Johnson was hired by campaign chairman Eli Segal and will start next Monday.
Johnson is a Minnesota native and a veteran of several Democratic campaigns for the Senate. He also worked on the presidential campaigns of former Nebraska Sen. Bob Kerrey (search) in 1992 and Walter Mondale (search) in 1984.
Graham, a Florida senator, exited the presidential race last month, saying he couldn't raise enough money to be competitive. Some Graham staffers went to work for Clark, the retired Army general who had entered the race shortly before Graham got out. Those staffers include spokesman Jamal Simmons and New Hampshire director Steve Bouchard.
Clark has been without a campaign manager since early October, when Donnie Fowler quit the job.