Mikell, 32, spent the past two seasons with the St. Louis Rams, starting 32 games and totaling 214 tackles, four sacks, two interceptions, seven passed defensed and nine forced fumbles.
Mikell said Carolina called on Saturday, he flew to Charlotte on Monday and "got thrown in the fire'' for the 12:15 p.m. practice. "I was kind of in shock because I didn't expect it,'' he said. "But I'm definitely excited.''
He was released in March in the second year of a four-year, $27 million contract signed following the 2011 season. He is expected to compete immediately with strong safety Mike Mitchell for a starting job.
The 5-foot-10, 205-pound Mikell is familiar to the Carolina coaching staff. He was an undrafted free agent out of Boise State when Carolina coach Ron Rivera was linebackers coach with Philadelphia. He was named to the Pro Bowl in 2009, when Carolina defensive coordinator Sean McDermott was an assistant coach with the Eagles.
Rivera said the decision to sign Mikell had more to do with injuries to Mitchell (calf) and Nakamura than it did with the performance of his safeties. He hasn't ruled out Mitchell, who injured his calf in practice last week and sat out of Monday's practice, playing in Sunday's opener against Seattle.
"You hate to get into a situation where come Friday neither one of the two are ready,'' Rivera said of Mitchell and Nakamura. "So we had to make a decision.''
Rivera said Mikell was a good fit because of his experience and because he played Seattle twice a year in each of the past two seasons with the Rams.
Mitchell said he plans to be the starter "until somebody tells me otherwise.''
Mikell has played in 155 regular-season games and has 91 starts. He has 742 career tackles, eight sacks and 12 interceptions.