Down slipped to a four-point defeat in their first home league game under new manager Jim McCorry on Saturday night.
The Mourne men played the last 25 minutes with 14 men after Luke Howard's dismissal, and also lost Kevin McKernan and Conor Garvey to black cards.
Roscommon hit an unanswered 1-4 to lead by 1-9 to 0-6 at the break.
Arthur McConville's palmed goal in the 46th minute brought Down back to within a point before Roscommon super sub Cathal Cregg added three late points.
Down started brightly with two Donal O'Hare points in the first three minutes but Roscommon hit four in a row in a six-minute spell, all from play, with Senan Kilbride turning onto his left to hit the best of them from long range.
The home side got back on level terms at 0-5 apiece through Paul Devlin's free, and briefly took the lead when Jerome Johnston, a sub for the injured Caolan Mooney, sent over his second point of the night.
From there, the Rossies completely took over and scored an unanswered 1-4 including a goal on the stroke of half time from Diarmuid Murtagh.
Kilbride's perfectly weighted pass over the top was well gathered by Murtagh, who finished it neatly to the far corner.
Kevin McKernan was black-carded for a trip after 25 minutes and Jim McCorry rang the changes, having made five of his substitutions by the start of the second half.
Things got feisty following Down's second black card for Conor Garvey and immediately after McConville's palmed goal in the 46th minute, Howard was sent off for a second yellow card.
After all the free-flowing football of the first half, the second was littered with bad-tempered spats and stoppages.
Points from McConville, Jerome Johnston and Peter Fitzpatrick brought Down right back into it and they closed the gap to a point, trailing by 1-11 to 1-10 with 10 minutes to go.
Despite having an extra man, Roscommon failed to score for 20 minutes until subs Cathal Cregg and Donie Smith relieved the pressure.
Down: S Kane; D O'Hagan, B McArdle, L Howard; P Devlin (0-2, 1f, 1 '45'), D Turley, K McKernan; N Madine, P Fitzpatrick (0-1); C Maginn (0-1), C Laverty, C Mooney; C Crilly, D O'Hare (0-3, 1f), D Savage
Subs: J Johnston (0-3) for Mooney (15), C Garvey for McKernan (BC, 25), M Poland for Savage (31), A McConville (1-1) for Crilly (HT), D O'Hanlon for Turley (HT), N Donnelly for Garvey (BC, 45)
Roscommon: D O'Malley; S McDermott, N Carty, N Collins; C Daly, N Daly (0-1), C Cafferkey (0-1); I Kilbride, C Shine; M Nally (0-1), D Shine (0-1, f), E Smith; C Murtagh (0-2), S Kilbride (0-1), D Murtagh (1-4, 2f) Subs: U Harney for Nally (12), D Smith (0-1) for Shine (50), C Cregg (0-3) for D Murtagh (53), M Whyte for O'Malley (53)
Referee: Barry Cassidy (Derry)