The DR Congo midfielder struck in the first half of a tight, breathless affair against a Leeds side who now have just one win in their past five games.
Whites manager Daniel Farke switched to three at the back on the hour mark in a bid to prise open a determined Baggies defence but, despite a late siege of the opposition goal, they could find no way through.
The home side could have won by more as Djed Spence kicked a Cedric Kipre shot off the line, although Leeds were denied a strong penalty claim when Kipre appeared to pull down Willy Gnonto as he shaped to shoot inside the box.
Both sides have been in stuttering form in December, but West Brom’s 1-0 win over Norwich on Tuesday hinted at their improvement, while Leeds fell to defeat at Preston as they watched the top three of Leicester, Ipswich and Southampton start to move further clear of them.
They began well enough but West Brom’s defensive improvement was evident as Kipre blocked Joel Piroe’s shot as he seemed set to score and Semi Ajayi superbly ran down and tackled Georginio Rutter as the visitors threatened again.
West Brom, with the outstanding Jed Wallace pulling the strings, played their way back into the game and Leeds themselves were grateful to big blocks from Liam Cooper and Ilia Gruev.
Leeds keeper Karl Darlow, standing in for the suspended Illan Meslier, was stranded as he ran off his line to try to meet Alex Mowatt’s corner, and when the ball fell to Kipre, his prodded volley was going in until Spence cleared.
Kipre then appeared to tug Gnonto to the floor after the Italy international had got goalside of the defender, but referee Graham Scott saw no offence.
Alex Palmer threw up a hand to deny Crysencio Summerville from curling one in from 20 yards, and when the ball switched to the other end, West Brom went ahead.
It was not surprising that Wallace, given acres of space by a malfunctioning defence, was the creator, racing away and pulling the ball back for Diangana. His shot was well blocked by a defender but the ball ricocheted back to Diangana and flew in off his knee.
Wallace went close to a second for West Brom and Tottenham loanee Joe Rodon headed a chance over the bar for Leeds, but Farke’s team looked a shadow of the side that had won five out of six in the autumn to state their promotion credentials.
Farke reacted by sending on Jaidon Anthony, Dan James and Patrick Bamford for Spence, Sam Byram and Gnonto, and Leeds perked up, penning Carlos Corberan’s side back for the first time in the match.
But West Brom have now only conceded three goals in five games, and they showed discipline and grit to secure their first win over Leeds in seven meetings.
“It’s three important points because we’re playing against an important opponent, and to win against an important opponent like Leeds you need to do a lot of good things.
“The thing that made me feel most pleased was the team commitment and the support of our stadium.
Today our stadium has been amazing from the beginning of the game until the last second of the game, and the players put in a lot of effort and personality to achieve the three points.
“It’s important to achieve a clean sheet and reduce the chances of the opponent and it’s something we still need to keep working on as in the first half there were some situations we should have managed better in defence.”
