With less than ten minutes remaining Ecuadorian winger Valencia cut in from the right hand side before rifling a shot across the face of goal, beyond the despairing dive of Paul Robinson and inside the far post.
And four minutes later substitute Young added an equally stunning strike, shimmying on the edge of the box before unleashing a fierce shot into the bottom corner to leave Blackburn floored.
Before that it had been a largely frustrating game for the Red Devils, unable to find the breakthrough despite enjoying large spells of possession.
Javier Hernandez came close twice for United in the early stages, first failing to connect properly when Wayne Rooney’s pass dropped perfectly to him and then striking a post when Valencia crossed from the right.
Steve Kean’s side were restricted to playing on the counter attack but used their opportunities to great effect forcing David De Gea to parry drives from David Hoilett and Marcus Olsson and tip over from Grant Hanley’s header.
Rooney came close to opening the scoring ten minutes after the break with a curling free-kick forcing Robinson into a diving save before a Morten Gamst Pedersen free kick called De Gea back into action.
But just when it looked like they might stumble and drop points in the title race United found the breakthrough at the crucial time to claim a seventh straight league win.
Sir Alex Ferguson’s side now sit five points clear of Manchester City with just seven games remaining. That means they can afford to lose to their local rivals and still become champions.
Blackburn meanwhile drop back into the relegation zone with QPR climbing out on goal difference.