The Baggies' goal difference meant they had effectively sealed promotion prior to their final-day visit to west London.
Goals from Kim Do-Hoen and Chris Brunt rubber-stamped their return to the Premier League and ensured they ended the season as champions.
The game swung in their favour after the 37th-minute sending off of Rangers captain Martin Rowlands, who was controversially shown the red card following a crunching challenge on Jonathan Greening.
Up to that point, QPR had looked marginally the better side with Gareth Ainsworth posing the main threat to Albion.
The veteran R's winger served up a fine cross for Dexter Blackstock, whose header was saved on the line by Dean Kiely.
Goalkeeper Kiely held a shot from Ainsworth and saw Rowlands' free-kick drift wide, while at the other end Rangers keeper Lee Camp did well to deny Brunt after the midfielder had been found by Greening.
Roman Bednar missed West Brom's best chance of the first half when he fired over from ten yards, but Rowlands' dismissal hit Rangers hard and it came as no surprise when the visitors did go ahead eight minutes after the break.
Zoltan Gera tricked his way past Damien Delaney and cut the ball back from the right, giving Korean midfielder Kim - on as a substitute - the easy task of heading home his first Albion goal.
Rangers never looked like recovering and Brunt hit the bar with a curling 25-yard effort before Gera fired just wide.
And any prospect of a Rangers comeback was crushed when Brunt hit the corner of the net with a fabulous 77th-minute free-kick.
The result sparked wild celebrations among Albion's 3,500-strong army of travelling fans, who saw their team presented with the Championship trophy after the final whistle.