The Tyrone legend showed some deft touches, teeing up his team-mates for scores and forging a lethal partnership with man-of-the-match Liam Gavaghan, and when they combined to give London the lead on 44 minutes they looked on course for victory.
However, with London pushing for a goal, they were twice hit at the other end as substitute Ronan Kennedy pounced for two goals in the space of three minutes.
A one-point lead turned into a five-point deficit and although London fought to the finish, they were unable to close the gap.
“We threw it away,” said Mulligan. “We trained hard for this. We believed in ourselves. I was only brought into the squad to help the forwards and then I end up having a crack myself and it was an honour for me to play for London.”
Leitrim looked on course for victory when Darragh Rooney latched onto Brendan Gallagher’s pass and goaled after 20 minutes. But London, inspired by Gavaghan, battled their way back and trailed by just points at the break, 1-6 to 0-7.
Leitrim went 16 minutes without a score while London hit six points without reply, Mulligan drawing the sides level on 43 minutes before then turning provider for Gavaghan’s go-ahead point. Rory Mason saw a shot come back off the post but London were hit at the other end as Kennedy twice punished naïve defending.
London tried their utmost to mount a comeback, with Brendan Flynn parrying a Gavaghan shot, and although they made it a two-point game with six minutes remaining they were unable to create a clear goal chance in the closing stages.
R Kennedy 2-0, D Rooney 1-2, D Moran 0-3, K Beirne 0-2 (1f, 1 ’45), D McGovern, B Gallagher (f), G Plunkett 0-1 each.
L Gavaghan 0-8 (2f), R Mason 0-4 (2f), C Doran 0-2, M Gottsche (f), O Mulligan 0-1 each.
B Flynn; M McWeeney, R Gallagher, P Maguire; J Rooney, D Wrynn, O Madden; S Moran, D Moran; D McGovern, B Gallagher, R O’Rourke; K Beirne, C Gaffney, D Rooney.
R Kennedy for Gaffney (39), W McKeon for R Gallagher (43), G Plunkett for D Moran (51), N Plunkett for Madden (55), C Cullen for S Moran (63).
G McEvoy; P Butler, C Coyne, C O’Neill; M Jordan, R Jones, Ciaran Dunne; A McDermott, L Gavaghan; M Carroll, M Gottsche, J Branagan; R Mason, C Doran, O Mulligan.
K Hughes for Doran (54), G Crowley for Carroll (54), D Carrabine for Ciaran Dunne (56), Colin Dunne for Jordan (66).
N Mooney (Cavan)