Thornton, who has played for Coventry Sphinx and Nuneaton Town this season, had been on a months contract at Sixfields but has done enough to earn a deal until the end of the season.
Cobblers Boss Ian Sampson told his club's website:"Kevin has done well and has impressed us. "He has earned his longer contract.
"He knows he has plenty of hard work ahead of him, but I think he is making good progress and we are delighted he will be staying with us."