After a goalless first-half at Craven Cottage, with the Magpies unable to record a shot on target in the opening 45 minutes, Fabian Schär looked to have broken the deadlock for the visitors after guiding the ball home from a tight angle but the goal was disallowed following a VAR review, with Dan Burn adjudged to push Calvin Bassey in the build-up.
Guimarães, however, sent United ahead in the final stages on his 100th club appearance, reacting to Harvey Barnes' cleared cross before firing a first-time effort beyond Fulham goalkeeper Bernd Leno to, ultimately, record a valuable three points in the capital.
Eddie Howe made two changes from the side which claimed a 1-1 draw against Everton on Tuesday night as Anthony Gordon returned to the starting line-up after serving a one-match ban while Joe Willock was also handed a start, with Harvey Barnes and Elliot Anderson both dropping to the bench.
Martin Dúbravka, targeting a sixth consecutive clean sheet against the Cottagers, was forced into early action as Fulham left-back Antonee Robinson fired an effort towards goal following Tosin Adarabioyo's lengthy run from distance but the American full-back's tame shot was comfortably held by the Slovakian goalkeeper.
The West London outfit created further opportunities in the opening exchanges of the league clash as Fulham skipper Tom Cairney dragged a curling effort wide of the target while Willian saw an effort blocked by Sean Longstaff before the Brazilian winger drove his rebound wide.
João Palhinha went close to sending the hosts ahead as the Portuguese midfielder latched onto Timothy Castagne's cross before firing a first-time effort narrowly wide of the far post.
Gordon was unfortunate not to score a tenth Premier League goal of the 2023/24 campaign as the Liverpool-born forward drove forward before whistling a low shot narrowly side from the edge of the penalty area.
The 23-year-old continued to cause problems for the Fulham backline, who dangerously advanced into the box after racing down the left flank before his resulting effort trickled inches wide of the target.
United's first shot on target arrived six minutes after the restart as the lively Gordon latched onto Isak's pass and after cutting inside from the left wing, saw his curling shot parried by Fulham goalkeeper Bernd Leno.
Dúbravka produced an important stop to prevent Andreas Pereira as the 35-year-old shot-stopper got down low to push away the South American's low drive from inside the box before the loose ball was hooked to safety by Lewis Hall.
Schär looked to have fired the Magpies in front as the Swiss defender connected with Gordon's cross at the back post before firing the ball into the net from an acute angle but referee Sam Allison was advised to review the decision before ruling out the effort after Burn pushed Bassey in the back before the ball reached his defensive teammate.
In the 81st minute, though, Guimarães hammered Newcastle into the lead. After Barnes' cross was cleared away by Robinson, the loose ball was lashed home by the Brazil international, who wildly wheeled away to celebrate in front of the travelling away supporters.
Despite the awarding of nine stoppage-time minutes, the visitors resisted Fulham's attacking pressure in the dying stages to record a ninth league clean sheet of the 2023/24 season.
