The Teesiders only had 10 men themselves, with Tony McMahon following Leeds' Max Gradel down the tunnel in the first half but, when home captain Jonny Howson also saw red, that was the invitation they needed to open the scoring.
Until Howson's departure, perhaps the most questionable of the three given that referee Anthony Taylor initially gave a foul on him for his second yellow card, neither side had looked capable of scoring barring a couple of close shaves.
The decisions merely added to an already tense atmosphere in the stadium, which had been generated by a pre-match protest by a section of the home supporters against chairman Ken Bates.
Their ire was soon directed on to the pitch as well, with Gradel and McMahon receiving their first yellow cards after nine minutes, the pair scuffling after Gradel had stretched into a tackle.
The Ivorian winger then warmed Carl Ikeme's palms with a 25-yard drive and whipped a free-kick over the bar, but that would be his last positive contribution of the match.
With 27 minutes gone he felled Rhys Williams as the Boro player ran through the Leeds half and, despite the protestations of manager Simon Grayson, he was sent back to the dressing rooms for the second time in his United career.
From the subsequent free-kick Emnes' header forced Andy Lonergan to make an acrobatic save, before Adam Clayton nearly scored for Leeds when his 45-yard lob almost caught out a scrambling Ikeme.
Boro then hit the woodwork, with Justin Hoyte finding room in behind Darren O'Dea to drill in an effort which Lonergan tipped against his bar.
With a numerical advantage, the north-east club were growing in possession and confidence, but that was eroded on the stroke of half-time as Taylor showed McMahon his second yellow for what at worst appeared to be a gentle lean on Howson.
Both sides had the chance to regroup at the interval and Leeds seemed to have used the opportunity better when the sides re-emerged, with Lees seeing a looping header from Robert Snodgrass' free-kick scrambled off the line by Joe Bennett.
Their momentum was abruptly halted, though, when Howson was sent off on the hour.
The Leeds captain had already been booked for an almost unavoidable foul on Emnes following a Michael Brown error but, when he and Emnes collided as the latter looked to break into a vacant Leeds half, Taylor deemed that Howson was the victim.
He soon changed his mind, though, and dismissed Howson, leaving Leeds with just one recognised midfielder on the pitch.
With ample space afforded to them Middlesbrough pushed for a goal and Scott McDonald thought he had it when a loose ball fell kindly for him, only for his fellow Australian Patrick Kisnorbo to hack his shot off the line.
However, Leeds could hold out no more and, in the 68th minute, Emnes finally got some space in the box and drilled a pinpoint shot into Lonergan's top corner.
The home fans quickly redirected their anger back to their chairman, while on the field Leeds were fortunate to only end the game one behind, with Lonergan twice saving well to deny Malaury Martin and McDonald, and then making a stunning injury-time stop to keep out Julio Arca.