Kyle Carragher's goal in extra time proved crucial as Crossmaglen won 2-17 to 2-12.
Substitute Brian McGinnity got a late goal for the Monaghan men to pull it back to 2-15 to 2-12 but Crossmaglen held on.
Scotstown had led early in the match thanks to a Darren Hughes goal, but Cross fought back to lead 1-6 to 1-3 at half-time with Tony Kernan getting their goal.
Scotstown's Kieran Hughes was sent-off for a high and late tackle and near the end his brother Darren was also red-carded.
Cross substitute Danny O'Callaghan was sent-off in the second half for an off-the-ball striking offence, but the Armagh team were restored to the full 15 men for extra-time.
James Morgan of Crossmaglen was also sent-off after picking up two yellow cards in extra-time but by that stage Rangers had the Seamus McFerran Cup in the bag.