Harmison, 33, is a former professional cricketer and is the younger brother of England cricket legend Steve.
He was released by the Premier Division runners-up at the end of the season.
Harmison said: “When I left South Shields, Morpeth was the only club I really wanted to go to, to be honest.
“It’s a club I have always admired and respected; the ambition for the future that the chairman and the management have is something I would love to be a part of. With the excellent squad we have, I’m really looking forward to hopefully bringing more success to Morpeth.
“My dad has also been a part of the club in the past – he was a coach when Keith Grant was manager years ago – so it is nice to follow in his footsteps.”
Also joining the Highwaymen is midfielder Michael McKeown, who arrives from West Auckland.
Assistant coach Jon McDonald said: “Micky has a great range of passing; he’s comfortable at receiving the ball under pressure and starting attacks. He will be a real asset for Morpeth.”
The duo join Sean Reid and Kris Thackray in joining the club, with David and Danny Carson and Jordan Fry departing.
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