France centre Wesley Fofana scored the only try just before the hour was up but the Pumas solid defence limited the hosts chances to score after they changed ends 15-3 down.
Argentina stunned the French by putting over four drop goals, with Sanchez's hat-trick and one from centre Juan Martin Hernandez. Sanchez also kicked two penalties.
France were fresh from a 40-15 win over Fiji followed by a 29-26 win over Australia while Argentina, having lost in Scotland two weeks ago, beat Italy 20-18 in their last outing.