Chester-born Dorman moved to the United States in 2000 and graduated from Boston University before beginning his professional career in Major League Soccer with New England.
St Mirren manager Gus MacPherson told the club's official website: "I'm glad to confirm that everything is now signed and sealed with Andy Dorman - he will be a St Mirren player in a few days' time.
"Andy has agreed to an 18-month contract with the option of a further year at the end of that.
"All that needs to be concluded is international clearance - we have requested that and it is now in the hands of the MLS in the USA to push the clearance through."
Dorman's contract with New England expires at the end of the month, and if international clearance can be obtained in time, his debut could come at Gretna on January 2.
MacPherson believes he has signed a high-quality player.
He explained: "We feel he is the type of player who will definitely strengthen our squad. He is a box-to-box style midfielder who can score goals - and technically he is very good."