The CFL club didn't provide a reason for the move but a league source requesting anonymity said it was to allow Mitchell to pursue NFL opportunities.
Mitchell, 28, is the fourth Argo player to be released this off-season to pursue an NFL deal, joining receiver Dontrelle Inman (San Diego), defensive back Neiko Thorpe (Oakland) and linebacker Marcus Ball (unsigned).
Mitchell's departure leaves a considerable hole on the Argonauts defence. He had 32 tackles, five sacks and a fumble recovery last season and was named an East Division all-star.
The Argonauts acquired the six-foot-five, 316-pound import in a trade with the B.C. Lions before the 2013 season. Initially, there were reports Mitchell wouldn't report to Toronto but he did show up and had an impressive training camp before his all-star season.
Prior to joining the Lions, Mitchell spent two seasons on the practice roster of the NFL's San Francisco 49ers.Suggest a correction