The former Rochdale forward also played for FC United, scoring 107 goals in 126 games for the club formed by Manchester United fans angered by the Glazers' takeover at Old Trafford.
Patterson signed a pre-contract agreement with Glentoran in January but he has left the east Belfast club without making an appearance.
Plymouth paid an undisclosed fee to Glentoran, who will also benefit from a sell-on clause on any future transfer and they have also negotiated a clause giving them first refusal on Patterson should he return to play football in the Irish League.
"Whilst we are obviously disappointed that we will not get to see Rory in a Glentoran shirt, the player made it very plain that he wanted the chance to pursue his dream of a full-time career in cross-channel football," said Glens chairman Aubry Ralph.
"The club felt it would be morally wrong to stand in his way of fulfilling that ambition.
"We have reached an agreement with Plymouth and we wish Rory every success with his move across the water.
"Rory wishes to place on the record his thanks to the club for facilitating the transfer to allow him to follow his dream."
The Strabane man's goalscoring exploits last season earned him the Carling Player of the Year award in the Irish League.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.