Down High School set their stall out from the start of the game and missed a penalty opportunity with less than a minute on the clock.
Five minutes into the game they scored the first points when Nathan Gibson took full advantage of a great lineout take by Thomas Donnan to touch down from the ensuing 25m rolling maul.
Midway through the first half Down gained good field position and Ricky Gill stormed through the Foyle defence to score close to the posts.
Luca Rea converted on this occasion to make it 12-0.
Neil Burns reduced the arrears to 12-3 when he converted a penalty chance for the Londonderry side.
Foyle started the second half in positive mood.
But 10 minutes in they missed a penalty opprotunity.
However, Burns did find his range again after 20 minutes.
Foyle scented a degree of concern in the Down ranks.
Centre, Ryan Curry, ran in for his fourth try in two games which Burns converted to put Foyle into a 13-12 lead for the first time in the game.
Down responded in the best possible way when Kelvin Hamilton ran in for an unconverted try to take the score to 17-13 in their favour.
Rea converted and it was 19-13.
The visitors secured their win with a well-taken try from Thomas McCavera who scrambled over the Foyle line in spite of the attentions of three would be defenders.
The conversion attempt was unsuccessful but the Down had secured a win and progression into the fourth round with the cup big boys.