The Smoggies squandered some good opportunities in the first half, either side of Aden Flint hitting his own crossbar, but struggled to truly get going in the second prior to their winner in added time.
Daniel Ayala, brought back into the side tonight for the first time this season in the league, blasted the ball past Sam Johnstone to take the Smoggies above Leeds United and three points clear at the summit after playing a game more.
A return to fitness for matchwinner Ayala allowed Tony Pulis to switch to a 3-5-2 formation of sorts against the side that sacked him nine months ago with the club one point above the Premier League relegation zone at the time.
An evenly-contested start to the match saw Jonny Howson's diving header skim the back of Britt Assombalonga and end wide of the target, shortly before a miscued Flint clearance came back off the frame of his own goal.
Creating chances from open play was proving far easier for the home side as the first half transpired, though, with Stewart Downing sending a shot down the middle for Johnstone to deal with and Howson then dragging wide after taking the ball past a couple of men.
Boro, winners in 12 of their last 14 Friday night fixtures, tightened their grip on proceedings in the final stages of the first half and created two more good opportunities.
Assombalonga failed to squeeze the ball in from close range following some slick play from Howson and Martin Braithwaite, and the latter blasted over an even better chance when the ball fell kindly to him seven yards out.
The Smoggies, also denied a penalty for Kieran Gibbs's tug on Flint's top, initially looked even brighter after the restart as Ayala's flick tested Johnstone and wing-back Ryan Shotton headed over a half-empty net under pressure from Gibbs.
Kyle Bartley was next to be denied a penalty after being dragged to the ground inside the box, while Jay Rodriguez's 35-yard shot was tipped around the post by Darren Randolph as the Baggies started to grow into the match.
A stalemate looked certain as both sides struggled to create much of anything in the final quarter of the match, but from nowhere Ayala struck a loose ball past Johnstone to win the match, although the centre-back appeared to use his arm to control it.
A second defeat of the season for West Brom, who are still without a clean sheet this term, leaves them just outside of the playoff zone.
MIDDLESBROUGH (3-5-2): Randolph; Flint, Fry, Ayala; Shotton, Clayton, Howson, Downing (Besic 73'), Friend; Braithwaite (McNair 86'), Assombalonga (Hugill 81')
WEST BROMWICH ALBION (3-5-2): Johnstone; Adarabioyo (Dawson 69'), Hegazi, Bartley; Gibbs, Livermore, Brunt, Barnes (Morrison 69'), Phillips; Gayle (Robson-Kanu 83'), Rodriguez