London delivered a typically determined display, but Roscommon still advanced to the Connacht Senior Football Championship semi-final.
John Evans' charges will face Sligo at Markievicz Park on June 20 following this win against a London side that produced a purposeful performance.
Paul Coggins' London were honest throughout, but Roscommon's attack were potent and their conversion rate was decent.
From 18 opening period scoring chances Roscommon posted 11 points which ensured they led at the break by 0-11 to 0-5.
Enda Smith struck four first half points for the visitors, who were bright and busy early on even though London stayed defiant.
Three early Roscommon scores from Donie Shine, Cathal Shine, and Smith set the tone, but London summoned a brave response.
Adrian Moyles and Alan O'Hara narrowed the gap with fine points, but Roscommon continued to move the ball swiftly.
Smith was forcing turnovers and the wing forward made them count as John Evans' side edged further ahead.
London were rocked by the 22nd minute black carding of Cathal Og Greene, who had been toiling diligently for the Exiles.
Despite Lorcan Mulvey's industry Roscommon still went in protecting a reasonable advantage at the break.
When Ciaran Murtagh scored a goal three minutes after the restart London were in significant bother, but five second half Lorcan Mulvey points kept them in touch.
Roscommon, though, were never in any serious danger despite the fact that London registered 15 wides during the match.
Roscommon: D O’Malley; S McDermott, N Carty, N Collins; D Ward, N Daly, C Cafferkey; I Kilbride (0-1), C Shine (0-2); E Smith (0-4), D Shine (0-2, 2fs), C Murtagh (1-2); D Murtagh (0-2, 1f), S Kilbride, C Cregg. Subs: R Stack for Ward (40 mins), C Connolly for S Kilbride (48 mins), U Harney (0-1) for E Smith (48 mins), D Smith for D Murtagh (52 mins), T Featherston for Carty (55 mins), R Daly for C Murtagh (60 mins).
London: D Traynor; B Collins, P Butler, D McGreevy; D McCarthy, C Carty, S Curran; M Gottsche, M Carroll; S Conroy, C Og Greene, J Moran; A O’Hara (0-1), L Mulvey (0-8, 5fs), A Moyles (0-1, f). Subs: P O’Shea for Greene (22 mins, black card), P McGoldrick for Moyles (44 mins), M Walsh for Moran (46 mins), A Carr for Carroll (54 mins), S Devine for McCarthy (55 mins), S Buckley for Conroy (69 mins).
Referee: E Kinsella (Laois).