Mitchell, 31, has made 66 starts since entering the league as a third-round pick out of Arizona in 2010, including 28 with the San Francisco 49ers over the past two seasons. He has also played for the Miami Dolphins and Houston Texans and has 6.5 sacks in 130 career games.
He became an unrestricted free agent after the 49ers declined to pick up his option for the 2019 season.
The Seahawks were expected to add a veteran defensive tackle after Reed's six-game suspension became official on Monday. Reed was deemed to have violated the NFL's personal-conduct policy during a 2017 incident in which he was accused of assaulting a woman at his home, though he was neither charged nor arrested in the case.
The Seahawks have been aware for some time that the NFL's investigation into the incident could result in a suspension and had a contingency plan in place. They brought in Mitchell for a visit during the spring. The Seahawks signed another veteran defensive tackle, presumptive starter Al Woods, in May and drafted Demarcus Christmas in the sixth round.
Also Wednesday, the Seahawks signed wide receiver Daniel Williams, a 2017 undrafted free agent out of Jackson State, and waived a pair of undrafted rookies: running back Marcelias Sutton and cornerback Derrek Thomas. Strong safety Lano Hill (hip) was placed on PUP and wide receiver Caleb Scott (foot) on NFI.
The Seahawks feel Ansah and his surgically repaired shoulder will be ready by close to Week 1, when Seattle opens at home against the Cincinnati Bengals. Dissly's rehab from surgery for a torn patellar tendon has gone well, and coach Pete Carroll has expressed optimism that he'll be ready for the opener.