The 29-year-old Johnson has five years of NFL experience, four with the Vikings. He is a former second-round pick from Arkansas State. Johnson started 27 games in Minnesota.
It unclear if Johnson will step into a starter's role immediately, but the Falcons have a need. Lowery suffered the third concussion of his NFL career and fourth since college during Friday night's scrimmage. The initial word is Lowery's season is in doubt despite a promising start to training camp.
Veteran strong safety William Moore worked with second-year safety Kemal Ishmael during Saturday's light practice. The combination is likely to continue over the next couple days as the Falcons practice in pads.
"Let me tell you something: Kemal is a heck of a player,'' Moore said. "He reminds me of myself: one of the underrated guys coming into the league with not much hype. But if you watch Kemal's ability, one day he'll gain the attention for people to watch his ability.
"Man, he came out here this year, and I don't know what he did in the offseason. But he came in lights out.''
The Falcons had been down two other safeties before Sunday, but rookie Dez Southward, who has been dealing with a left knee injury, was back on the practice field with a helmet. Second-year safety Zeke Motta, however, will likely miss the entire season coming off neck surgery.
To make room for the veteran Johnson, the Falcons released fullback Roosevelt Nix-Jones.