Leeds United 0 Sheffield United 4
Once a member of the Leeds Academy, Montgomery was shown the door and ended up at Bramall Lane, so there was an extra flourish as he drove home the Blades' second goal.
Sheffield United had gone ahead in the second minute through Danny Webber and another former Leeds player, Andy Gray, fired home the third two minutes into the second half and rounded off his return to Elland Road with a second 15 minutes from the end.
And to add to Leeds' misery something was thrown from the crowd at the linesman, which could well see the cash-strapped club fined, and skipper Paul Butler was stretchered off after being injured in the build up to Blades' fourth goal.
Webber took less than two minutes to make an impact on his first start for the Blades after his loan move from Watford, drifting past Butler and Simon Walton and firing inside Neil Sullivan's near post.
With Yorkshire bragging rights and a possible play-off spot up for grabs, both teams need to win this but Kevin Blackwell's side seldom looked likely to threaten Paddy Kenny's goal.
Their first three efforts, from Danny Pugh, Seth Johnson and Rob Hulse, were all from 25 yards or further and none of them were close enough bring the Blades keeper into action.
The visitors were chasing and harrying every ball and when they moved forward, Leeds young defence always looked fragile, especially teenager Walton, playing only his second game at right-back and clearly uncomfortable in that position.
Sheffield's second goal inevitably came down their right flank, but while Walton could be faulted for letting Alan Quinn get to the by-line and cross, the rest of the Leeds defence shouldn't have allowed it to go low right across the face of goal. Montgomery had the easiest of jobs to force the ball in.
Blackwell put Walton out of his misery in the 39th minute, sending on Sean Gregan into midfield, with Shaun Derry switching to full-back. Gregan, just back from a two-match ban, picked up his 12th booking of the season within two minutes.
There was some joy for the restless home fans in first-half injury time when Hulse created some space in the box, but his fierce shot hit the underside of the bar and came back out.
Any hope Leeds had of coming back were killed off two minutes into the second half when Phil Jagielka won a header far too easily and set up Gray's shot into the far corner.
Leeds suddenly burst into life and started to take the game to the Blades and it took a magnificent instinctive save from Kenny to stop Johnson's close-range effort after good work by Aaron Lennon.
At least they were showing some passion but with 15 minutes to go, a misunderstanding between Butler and Sullivan left the Leeds captain laid out and Sullivan vainly trying to prevent Gray's second.