Sunday's move comes after safety Dwight Lowery left Friday night's scrimmage with a concussion. Lowery missed most of the 2013 season with Jacksonville after suffering a concussion at Seattle on Sept. 22. He later was placed on injured reserve and released.
Johnson, a second-round pick by Minnesota in 2008, played four seasons with the Vikings. He played four games with Detroit in 2012.
The Falcons waived fullback Roosevelt Nix-Jones to clear a roster spot for Johnson.