The 26-year-old, who has also represented Walsall, began his career at hometown club Wolverhampton Wanderers and made two first team appearances at Molineux.
He has agreed an initial one-year contract with the Pilgrims, which will officially kick in following the expiry of his deal at Field Mill.
Hemmings is the ninth signing of the summer at The Jakemans Stadium and manager Adam Murray was overjoyed to land a long-term target.
Murray said: "It's a great signing for us. He's somebody that I know well and who did well for me at Mansfield.
"He had other options and we had to wait and be patient. We have tried to sell our football club to Ashley and he has bought into that. He wants to be part of what we are trying to build.
"We need people like Ashley onboard if we want to be successful."