The Sheffield born Crystal Palace boss, who has supported United since he was a schoolboy, looked on helplessly as his in-form Eagles were torn apart.
Palace, with just one defeat in eight games, were in trouble from the start.
They escaped after £3million striker Ched Evans was given too much space and his fierce left-foot strike from 10 yards deflected off defender Danny Butterfield and into the side-netting.
Jamie Ward surged through from midfield minutes later but, with Palace slow to close down, the little winger scuffed his shot from the edge of the area and Julian Speroni saved comfortably.
United's snatched the lead in the 21st minute when Evans picked out Stephen Quinn.
The visitors failed to cut out his low cross as Williamson nipped in to stab the ball over the line as Speroni made a vain attempt to claw it away.
Speroni foiled the Sheffield side as he stretched to palm away Ward's inswinging corner before diving to foil Evans as he drilled in the loose ball at the far post.
On-loan Stoke striker Richard Cresswell's fierce strike was deflected before Williamson almost grabbed a second after lashing the ball inches over the target after Chris Morgan had headed Quinn's free-kick back into the danger area.
Cresswell steamed in to pick up Quinn's low cross from the right flank but fired wide from a tight angle, while Palace rarely troubled home goalkeeper Mark Bunn.
Neil Danns fired wide with a long-range strike in a rare foray into the United area before Quinn killed off Palace with a low strike in stoppage time.