The former Arsenal midfielder's second half strike settled a tight contest in which Willock's terrific finish - just six days on from his goal in the 4-1 thrashing of Southampton - sent St. James' Park into raptures.
The result leaves Eddie Howe's side just four points shy of leaders Arsenal, for a few hours at least, going into the World Cup break.
In an attritional first period, chances were few and far between. Newcastle were typically bright and probing, with their best opening falling to the in-form Miguel Almirón. The Paraguayan - October's top flight player of the month and goal of the month winner - saw a volley drop harmlessly over the crossbar after some good work from Willock down the left. His cross almost found Chris Wood in the middle before dropping for Almirón, who couldn't keep it down.
Chelsea, meanwhile, posed few questions in the final third. Their only attempt came courtesy of Armando Broja, whose smart turn allowed him to fire low at Nick Pope, but the shot lacked power. The visitors survived a penalty appeal in the half's closing moments, too, when Trevoh Cholabah stopped Dan Burn's low cross with his arm.
United rallied after the restart, with Sean Longstaff seeing his finish from Joelinton's knockdown saved before Bruno Guimarães' follow-up was blocked. Longstaff then hooked over seconds later as the hosts fizzed with urgency. Willock's inviting cross narrowly evaded the sliding Wood as Chelsea struggled to keep a grip on the Magpies' attack.
But at the other end, they grew in confidence. Mateo Kovacic saw his drive charged down before Nick Pope, at full stretch, produced a magnificent one-handed stop to turn Conor Gallagher's curling strike round the post.
The visitors were made to rue that save just a few minutes later. Joelinton's looping forward ball was picked up by Almirón, who cut infield. When he ran out of space he nudged it into Willock's pass and his shot was unerring and perfectly struck, beating Edouard Mendy and nestling in the top right-hand corner.
Chelsea looked to respond by sending on Hakim Ziyech, Kai Havertz and Marc Cucurella, with the latter's low cross causing some brief alarm in the United six-yard box. But Newcastle - for whom Callum Wilson made a late substitute appearance before his, Pope and Kieran Trippier's departure for the World Cup with England - were good value for their victory.
On the same day in 2021, Newcastle sat winless in 19th place. Saturday's victory over the Blues kept Howe's men in third place and just four points behind the table-topping Gunners, who face Wolves in Saturday's late kick-off. It has been a remarkable 12 months on Tyneside.
