Mitchell said after the season he hoped to return to Carolina, and the Panthers were hoping to re-sign him once the free agent market settled down if the price was right.
It never got to that as Pittsburgh, one of several teams pursuing Mitchell, reached a quick deal, signing him to a five-year $25 million contract, according to sources.
Mitchell took to Twitter to thank the Panthers after his deal with the Steelers was reached.
I would also like to thank Mr. Richardson and the entire Panthers organization for their support this season.
- Mike Mitchell (@MikeMitchell34) March 11, 2014
Mitchell set the tone for the league's No. 2 defense with his aggressiveness when he replaced Charles Godfrey after the free safety suffered a season-ending Achilles injury in the second game at Buffalo.
He had a career year in starts (14), tackles (74), interceptions (4), sacks (3.5) and forced fumbles (2).
Mitchell signed a one-year deal with Carolina after spending the previous four seasons at Oakland, where he started only nine games after being selected in the second round of the 2009 draft out of Ohio.
Mitchell was one of the more outspoken members of the team, as well as most penalized. After being fined $7,875 for taunting against the St. Louis Rams, he called out NFL commissioner Roger Goodell for making him a target.
Mitchell said it was his fifth fine of the season, totaling more than $45,000.
He ended the season by calling out the officials in Carolina's 23-10 loss to San Francisco in the NFC divisional playoff game. He disputed an unnecessary roughness call against him in the first quarter that led to a 49ers' field goal and cited officials for being inconsistent with the head-butt, again costing the Panthers.