The Dragons were left to rue their indiscipline after Magners League title-chasers Leinster snatched victory at Rodney Parade.
In a fiery game that saw five players sin-binned, Felipe Contepomi scored 13 points before creating Chris Whitaker's late try to ensure the Gwent region's first defeat in ten home games.
A brace from Adam Black and a first-half touchdown from hooker Steve Jones wasn't enough after the brilliance of Contepomi boosted Leinster's championship hopes.
Brain Blaney and the boot of Contepomi had kept the league leaders in the hunt as Leinster trailed 15-10 at the interval. The second-half was a tight affair but in the final quarter of hard-fought game, Dragons pair Ian Gough and Ceri Sweeney both saw yellow in as many minutes.
Argentine fly half Contepomi made them pay with his third penalty before his break freed replacement scrum half Whitaker to cross in the closing stages.
Black set up a tense finish with his second try in injury time to salvage a Dragons bonus point, but it still wasn't enough to deny the Irish side a vital victory.
Dragons: Tries: Jones, Black (2). Cons: Sweeney, A Thomas. Pens: C Sweeney
Leinster: Tries: Blaney, Whitaker. Cons: Contepomi (2). Pens: Contepomi (3)
Dragons: M Thomas (A Thomas 55); G Wyatt, P Emerick, N Brew, A Brew; C Sweeney, G Cooper; A Black, S Jones (B Daly 62-69), L Harrison (R Thomas 11), I Gough, L Charteris (P Sidoli 72), C Charvis, M Owen, J Ringer
Leinster: G Dempsey; S Horgan, G D'Arcy, R Kearney, D Hickie, F Contepomi, G Easterby (C Whitaker 39); R Corrigan (S Wright 53), B Blaney, W Green, T Hogan, M O'Kelly, S Keogh, J Heaslip, K Gleeson