Leitrim struggled for long periods against lively visitors who never gave up in Carrick-on-Shannon yesterday.
The home side were ring-rusty following a four-week lay-off while London looked sharp and scored first through Eoin O'Neill.
James Glancy levelled, but the Exiles deservedly led by 0-4 to 0-2 after 24 minutes as Leitrim repeatedly lost hard-won possession through poor passing.
Then three minutes from the break came the game-changer when Glancy found Ray Cox in space and the corner-forward finished to the net to leave Leitrim 1-4 to 0-4 ahead at the interval.
Glancy extended the lead eight minutes after the resumption and although the sides exchanged points during the third quarter, Leitrim pulled away with scores from Glancy, Darren Sweeney, and a brace from Cox.
Man of the Match: James Glancy (Leitrim)
SCORERS -- Leitrim: R Cox 1-3 (0-1f), J Glancy 0-4 (1f), D Lowe, P McGowan, D Sweeney (1f) 0-1 each. London: E O'Neill 0-3, S Doolan, S Kelly, M Mullins, P McDermott, E Mageean 0-1 each.
LEITRIM -- C McCrann 6; A Wynne 6, C Egan 6, P Maguire (capt, 6); D Beck 6, C Sheridan 6, W McKeon 6; D Sweeney 7, P McGowan 7; P Brennan 6, R Lowe 7, G Hickey 6; A Croal 7, J Glancy 8, R Cox 8. Subs: T Beirne 6 for Croal, D Gilhooley 6 for Clancy, C Ryan 6 for Hickey.
LONDON -- E Beirne 6; S Hannon 6, S McVeigh (capt, 7), D McGreevy 6; T Gaughan 6, S Mulligan 6, B Kerr 6; A Raftery 7, L Mulvey 7; F McVey 6, P McDermott 6, S Doolan 6; M Mullins 6, E O'Neill 7, P Sherry 6.
REF -- S McLaughan (Donegal).