Boro had failed to pick up a point in the new year before this match and had seen their Championship play-off place come under increasing threat from a number of chasing clubs, which Leeds considered themselves to be one of.
But with United struggling with form issues of their own and finding themselves in the midst of another confusing takeover bid for the second time this season, they were unable to present a consistent threat and suffered their first defeat at the Riverside since it opened in 1995.
Come the end, their fans were chanting 'Warnock, time to go' at their under-fire manager Neil Warnock, while in the opposition dugout Tony Mowbray was left celebrating, even in spite of Main's dismissal four minutes after his 81st-minute goal.
The striker's back-post header from Ishmail Miller's cross was reward for Middlesbrough's second-half improvement, although they had to weather a slight first-half storm as an unchanged Leeds impressed early on.
After Rhys Williams' early chance for Boro, David Norris hit wide and then warmed the palms of Jason Steele after some neat interplay between Ross McCormack and Steve Morison, while the in-form Luke Varney also drove away from goal after cutting in from the left.
On 28 minutes, Paul Green played a hopeful long ball forward that was Steele's all day long but he got his feet wrong and slipped, leaving McCormack with an open goal six yards out. The Scotland international dallied, though, and when he got a shot away, Jonathan Woodgate had recovered sufficiently to deny his former club on the line.
That proved to be Leeds' last chance of the first half and they should have been behind by the time the interval came. Although not as clear a chance as McCormack's, Faris Haroun got clean through in injury time off Josh McEachran's defence-splitting pass, but Paddy Kenny was quickly off his line to deny him on the edge of the area.
The first half had hardly been an end-to-end affair but it was in comparison to the start of the second, which was attritional at best.
Woodgate provided the best moment, showing impressive pace and awareness to block off McCormack's run to goal, but as has often been the case in the 33-year-old's career, injury forced him off midway through the half.
Leadbitter, the game's other class act, then curled an edge-of-the-box free-kick just wide of Kenny's right-hand post and that brought about a mini period of drama with Leeds forced to defend for their lives with 17 minutes left.
Kenny did well to deny Leadbitter down to his right and then combined with Sam Byram to deny Williams on the follow-up, with the keeper then darting across his line to pouch a McEachran header that should have been a forgone conclusion.
Boro seemed keen to take the initiative, though, and by bringing on Miller they took it. With nine minutes left, his cross hung enough for Main to put a header back across Kenny's goal and into the top right-hand corner.
He was then dismissed by Keith Stroud for a combination of dissent and kicking the ball away but, even though Steele had to deny Habib Habibou, Boro were able to hold on without him.