The tight-head's arrival was announced by head coach Kingsley Jones at a supporters meeting at Rodney Parade.
He said: "He's put his hand up, he still wants to play for Wales and I think it's a great signing for us.
"We all know tight-head is a critical position and that he will bring a lot."
Mitchell brings extra competition at tight-head and will be in competition with Lloyd Fairbrother, Shaun Knight, Leon Brown and current incumbent Brok Harris for the No.3 shirt.
His Cardiff Blues side face Dragons in Round 20 of the Guinness PRO12 on Sunday but Mitchell is unlikely to feature after being kept out of the side by Dillon Lewis, Salesi Ma'afu and Taufa'ao Filise.
Jones also said at the meeting that he hopes wing Hallam Amos will commit his long-term future to the region and that several young players have also signed on the dotted line.
He added: "Hallam Amos is contracted until 2017 and hopefully we will get him secured on a long-term deal very shortly.
"Tyler Morgan, Elliot Dee and Angus O'Brien are committed until 2018.
"Ashton Hewitt, Matthew Screech, Ollie Griffiths, James Benjamin, Dorian Jones, Leon Brown, Harrison Keddie, Cory Hill, George Gasson, Keagan Bale and Connor Edwards have all committed until 2019 or 2020.
"We have all seen the young players we have. They have massive potential, we need to keep them here until they have reached their full potential."
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Guinness PRO12 Dragons News
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