Stephen Warnock was sent off for conceding a late free-kick and when Royston Drenthe flicked on from the set-piece, Le Fondre pounced to undo his old boss.
McDermott - sacked by Reading in March after more than 12 years at the club, mostly as chief scout but also as a title-winning manager - received a warm welcome back from the home fans, as did former striker Noel Hunt.
The pair had nothing to celebrate, though, as Nigel Adkins' Royals got a win they just about deserved.
The home side recovered from a poor start, though, with midfielder Jem Karacan carried off on a stretcher after just nine minutes, appearing to twist his right knee when unfairly challenging Ross McCormack.
It carried unfortunate shades of the past for the midfielder for it was against Leeds in April 2012 that he broke his ankle.
Rodolph Austin fired over the free-kick that Karacan had conceded and, with Hope Akpan getting early action from the bench, Reading took a while to regroup.
When they had done, Paddy Kenny saved superbly from a 20-yard piledriver from Danny Williams.
Hal Robson-Kanu also tested Kenny from distance and, again, the Leeds goalkeeper was up to the task. But the game became increasingly scrappy, with Reading picking up bookings for Danny Guthrie and Alex Pearce for clumsy fouls.
Little was seen of a lightweight Leeds attack, either from Hunt or Luke Varney.
They did, though, have scant support from midfield or elsewhere. When Varney did find space, he launched a weak effort at Alex McCarthy in the home goal.
Austin's 25-yard attempt carried a lot more power but lacked direction as it thundered into a stanchion behind then net. Reading replied with carbon copy Sean Morrison headers from free-kicks - the first from Danny Guthrie, the second from Drenthe - but he nodded both set-pieces high and wide.
Amazingly, Morrison got another chance early in the second half, this time from a Drenthe corner, but the result was the same. He failed to get over the ball and nodded wildly wide.
Leeds sat back unadventurously, as they had done for large portions of the first half, but at last got going when Hunt drove tenaciously for the byline. But he ran out of room and McCarthy comfortably gathered his cross.
Soon after, Hunt was replaced by Dominic Poleon.
Reading upped the ante again when a Nick Blackman drive seemed goalbound until it took a deflection off Warnock.
Robson-Kanu then squandered the best opening of the match when he chose to shoot from 20 yards instead of passing to team-mate Blackman, who was unmarked. Robson-Kanu blazed over and Blackman shook his head in dismay.
It had looked like a draw would be played out at that stage, but when Warnock tripped Le Fondre for a second yellow, Daniel Williams' free-kick was flicked on and Le Fondre did the rest.