The deal was announced Monday. Contract terms were not released.
Mikell, a 10-year NFL veteran, will compete for the starting spot with strong safety Mike Mitchell, who was held out of practice Monday with an unspecified injury.
The 5-foot-10, 205-pound Mikell started all 32 games the last two seasons with the Rams and had 214 tackles, four sacks, two interceptions and forced nine fumbles. Mikell went to the Pro Bowl in 2009.
The 32-year-old Mikell previously played for Panthers coach Ron Rivera when Rivera was an assistant with the Philadelphia Eagles in 2003.
Mikell has played in 155 career games with 91 starts.