United's win against West Brom, coupled with Manchester City's shock loss at Swansea on Sunday, was enough to send Sir Alex's men to the summit of the Barclays Premier League.
With 10 games to play, the Reds are one point clear of neighbours City, whose next test comes when Chelsea visit the Etihad Stadium on Wednesday 21 March. By that time, though, United could be as far as four points ahead, following a trip to Wolves on Mother's Day.
On Sunday in Wales, City slipped to a surprise defeat at the Liberty Stadium courtesy of a late Luke Moore header. The home side had earlier squandered a chance to go ahead from the penalty spot, with Joe Hart saving Scott Sinclair's weak penalty.
City thought they'd rescued a point when Micah Richards found the net on 88 minutes, but the goal was correctly ruled out for offside and, at the final whistle, Roberto Mancini's men had been toppled from top spot.
Moments after Swansea went ahead, a huge cheer went up inside Old Trafford as word from Wales filtered through to Manchester.
"We knew Swansea had scored," Wayne Rooney told MUTV. "We didn't know if it was 1-0 or 2-0 but we knew they'd scored. We just had to stay focused and make sure we kept playing our game and see the game out.
"There's a long way to go but it's nice to get to the top of the league and be looking down. Hopefully we can stay there now."