Leitrim will face Mayo in the Connacht SFC semi-finals after eclipsing London in the second half at Ruislip for a 0-12 to 0-08 win.
In wet and windy conditions, the Exiles got on top with Padraig McGoldrick’s 32nd-minute goal giving them a 1-05 to 0-05 half-time lead. But Leitrim kept them to just three second-half points as man-of-the-match Emlyn Mulligan led the visitors’ comeback.
Substitute Shane Moran brought them level in the 67th minute and further scores from Mulligan and James Glancy sealed it.
They made London pay for some poorly-taken frees earlier in the game when they failed to take full advantage of a strong wind at their backs.
Leitrim joint manager Barney Breen and George Dugdale used their bench intelligently and a number of those substitutes made an impact, including former Longford centre-back Enda Williams.
Paul Coggins’ London side were unable to push on after McGoldrick’s timely strike. Tellingly, they have struggled for goals this season with only two in their previous 11 outings in 2012.
Leitrim fielded two Championship debutants in their starting line-up with Melvin Gaels’ Fabian McMorrow selected at centre-back, while Paddy McGowan got the nod at right wing forward.
London opted to play with the elements having won the toss, and they were first off the mark thanks to a second minute effort from Mark Gottsche.
After a couple of wides from the Exiles, Leitrim’s danger man Emlyn Mulligan linked well with James Glancy but was short with the subsequent free.
Sligo man Padraig McGoldrick was wide with an eighth-minute free for London, with both sides struggling to create space in a tense rain-hit opening.
Leitrim had to defend for long spells of the first half with the Exiles dominating possession and they deservedly added a second point through free-taker Lloyd Colfer, the liveliest of their forwards.
But either side of that Emlyn Mulligan grabbed a brace, bringing the visitors level from a 19th minute free.
Poor shooting let London down as the conditions worsened and Leitrim, continually threatening on the counter attack, nipped ahead with a confident score from McGowan.
Another purposeful break out of defence led to Glancy picking off a fantastic fourth for Leitrim and a 30-metre free from the influential Mulligan made it 0-05 to 0-02.
But London wrestled back control of proceedings before the break. The key moment for them came when a high ball in caused problems for the Leitrim defence and McGoldrick pounced for his goal.
McGoldrick was substituted soon after but his goal seemed to energise Coggins’ charges and further points from Eoin O’Neill and Colfer widened the margin to three - 1-05 to 0-05.
London wing forward Sean Kelly claimed the first point of the second half, but that was cancelled out by Mulligan who profited from good work by substitutes Enda Williams and Robbie Lowe.
Williams, making his Championship bow for his adopted county, popped up in attack to slot Leitrim’s seventh point and suddenly the hosts were being pinned back.
Their defence, marshalled by Antrim-born captain Sean McVeigh, was having to get through more work as Leitrim benefited from the introduction of those fresh legs.
Colfer notched a much-needed point for London from a free, but the gap was down to the minimum (1-07 to 0-09) by the midpoint of the half.
Ray Cox and Williams helped set up Glancy for his second of the afternoon and midfielder Darren Sweeney broke forward to open his account.
Now creating more scoring opportunities, Leitrim pressed for the equaliser and were left frustrated by two misses from Mulligan which took their wides tally to 11.
However, young substitute Shane Moran got them back on track with a terrific score with just three minutes of normal time remaining.
With London unable to respond, Wayne McKeon was brought down for a free which Mulligan right-footed through the posts for the lead score at 0-11 to 1-07.
From the kick-out, Leitrim teed up Glancy for what was effectively the match winner. It was a point they needed in the end.
London completed the scoring with an Eoin O’Neill free in injury-time, but there was no final flourish as their search for a first Connacht SFC win since 1977 goes on.