Carolina's previous three pickups were defensive end Tyler Brayton, cornerback Ricardo Colclough and guard Keydrick Vincent.
The Johnson deal comes one week after the free-agent market opened and less than 24 hours after he completed a visit to Bank of America Stadium.
"He met with the team and was thrilled with the organization," said agent Andy Simms. "When he got back to Cincinnati we talked about the visit and he really wanted to get a deal done with Carolina. He was excited to become a Panther."
Johnson, who turned 27 on Thursday, has spent the past four seasons with the Bengals and played all three linebacker positions. He started 53 of a possible 64 games and had 109 tackles in 2007 and 112 in 2006.
It's likely the Panthers will want him to start at weak-side linebacker where Na'il Diggs played last season.
Another option might be moving strong-side linebacker Thomas Davis to safety, although that seems unlikely given his progression at linebacker last season.
If Davis remains at linebacker it gives the Panthers four starting caliber players in Davis, Johnson, Diggs and Jon Beason.
The team also has James Anderson and Tim Shaw as backups, but they may lose restricted free agent Adam Seward to the New England Patriots. If the Patriots sign Seward to an offer sheet the Panthers would have a week to match. If not, they would get New England's fifth-round draft pick as compensation.
Where does Johnson fit in?
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