The former Bengal flew to Detroit after being released and impressed the Lions with his workout and conditioning. Following Johnson's visit, there was a buzz in the Lions' locker room among players who saw Johnson's workout.
Johnson had one of his best offseasons, trainingwise, but he missed a good portion of training camp due to a hamstring injury.
Because the move was made after 4 p.m. ET Monday, the Lions didn't have to make a roster move until Tuesday. Tatum Bell, who lost the starting job to rookie Kevin Smith, was released on Tuesday to make room for Johnson.
The 5-foot-10, 225-pound Johnson rushed for 5,742 yards in seven seasons with the Bengals. He ran for a team-record 1,458 yards in 2005 and 1,300-plus yards in '06.
Smith is expected to begin the season as the Lions' every-down starter, but Johnson will compete for the job once he learns the offense.
The Lions shouldn't have any problems accommodating a two-back system -- they plan to run the ball a lot this season.
Bell came to Detroit before last season in a trade with the Broncos. He had 44 carries for 182 yards in five games for the Lions.
John Clayton is a senior writer for ESPN.com. Information from The Associated Press was included.