The Giants signed Thornton to a contract Wednesday, Michael Eisen of the Giants' official site reports.
Thornton assumes the roster spot of cornerback Janoris Jenkins (ankle), who was placed on injured reserve after undergoing season-ending surgery Wednesday. A third-round draft pick of the Lions in 2014, Thornton has appeared in 23 games with Detroit over parts of the last three seasons, producing 28 total tackles and one sack. He'll serve in a depth role along the defensive line in New York.