Mitchell, 30, arrives at Croft Park having made over 160 appearances for Scottish League Two side Annan Athletic.
He started his career in Carlisle’s youth academy and was a member of the squad that famously beat Manchester United in the FA Youth Cup in 2007/08.
After leaving Brunton Park, Mitchell headed north of the border to Gretna before going on to enjoy a long and successful spell with Annan.
Mitchell was voted into the Annan supporters’ team of the decade last year.
While travelling back and forth between Cumbria and Scotland, Mitchell also has played for Workington Reds.
Indeed, Mitchell was the man in between the sticks against Spartans when the visitors ran out 4-3 winners during the Northern League Premier League play-off semi-final in April 2016.
Some supporters may remember the spectacular save he pulled off to keep out Nathan Buddle’s header in the final minutes of that game.
Mitchell’s arrival fills a key position for Blyth boss Michael Nelson, who, despite a manic few months of signings, had yet to recruit a goalkeeper.
He said: “Alex is someone we identified and also had recommended by two separate goalkeeping coaches that we trust.
"He comes with a wealth of experience in the game and a good number of games under his belt. He is extremely agile and an excellent communicator with the players in front of him.
"We are delighted to have him on board and look forward to working with him during the course of the season.”