Mitchell, who'll turn 27 on June 10, was a surprising second-round draft pick by the Oakland Raiders out of Ohio University in 2009, but he didn't enjoy a breakout season until last year when he moved into the Panthers' starting lineup.
Last year, the 6-foot, 213-pounder from Fort Thomas, Kentucky, started 13 games and had 4 sacks, 4 interceptions, 2 forced fumbles, 10 passes defensed and made 52 tackles, according to Pro Football Reference.
In four previous seasons with the Raiders, Mitchell played in 61 games but made only 9 starts and had 3.5 sacks, 2 interceptions, 1 forced fumble, 12 passes defensed and made 87 tackles.
Mitchell was considered by his Panthers teammates as something of a ballhawk with his 4 interceptions, but he was also the recipient of a pass rush that compiled 59 sacks.
The Steelers, who had only 10 interceptions as a team last season, compiled only 34 sacks as a team.
Mitchell's arrival spells the end for Ryan Clark, a free agent who has not received an offer from the Steelers. Clark took the news well and tweeted "Congrats to Mike Mitchell and the Pittsburgh Steelers!! It has been a pleasure playing for 1 of the best organizations in sports!! God bless."
Mitchell's tweet was understandably more celebratory.
"Pittsburgh!!!!!!," he wrote. "First I have to thank God for this blessing and Mr. Rooney and the Steelers organization for such a great opportunity!"
Ohio, Mitchell started 23 games at strong safety and 7 games at free safety (all as a senior) and had 7 career interceptions.
He wasn't invited to the combine, but at his pro day had reported times of 4.39 and 4.43 in the 40. He also had 22 bench reps of 225 pounds, a vertical jump of 37.5 inches, a broad jump of 10-3 and a three-cone time of 6.85.
He was drafted with the 47th pick by the Raiders, 15 picks after the Steelers chose Ziggy Hood at the end of the first round.