A bad-tempered affair looked to have been settled by Jason Kennedy's well-taken goal, but McGleish made no mistake from the spot after Matt Harold was felled in the area deep into injury time.
A largely lacklustre encounter sparked into life after 80 minutes when Darlington's Tim Ryan and Chris Zebroski of Wycombe were both sent off. Zebroski's red card appeared to come after he lashed out at one of his own players.
In truth, it was exactly what Wycombe needed to wake them up.
The league leaders had been sluggish in the first half and surrendered possession all too easily. However, they should still have taken the lead when Zebroski fired wide when well placed six yards from goal.
Darlington missed their best chance of the half moments later when Billy Clarke drove his shot across goal from the left channel.
The Quakers took a deserved lead 12 minutes after the break. Neil Austin did well on the right before squaring for Kennedy who drilled his shot low and hard beyond Scott Shearer.
The goal should have galvanised the home side, but they still looked toothless going forward.
The double sending-off opened the game up for the final quarter hour and Wycombe began to throw bodies forward.
However, it looked like Darlo would hold on to the points until Stephen Foster brought down Harold in the area seven minutes into added time.
McGleish gleefully powered his spot-kick past Simon Brown to keep Peter Taylor's side top of the pile in League Two.