The former Luton forward has joined on a two-year deal with an option for a 12 month extension following his release by Barnet.
Leary, who teams up with Mike Newell once again after playing under the Mariners boss at Kenilworth Road, used to watch Grimsby with his father as a youngster and will be fulfilling a long-held ambition when he finally runs out at Blundell Park.
He told the Grimsby Telegraph: "I'm delighted to be at Grimsby. I've got connections because I've got family here.
"Some people dream about playing for Man United or Liverpool, but I always wanted to play for Grimsby because of my family so it's a bit of a dream come true.
"I think my dad, uncle, auntie and cousins will be made up with me joining."