Harvey Barnes' neat finish, the winger's 50th Premier League goal, fired the Magpies into a first-half lead in West London before Kevin Schade's close-range header drew the Bees level before the hour mark.
A dramatic conclusion followed as United fell behind through Thiago's spot-kick, with Dan Burn receiving a second yellow card after being adjudged to bring down Dango Ouattara inside the box, before Thiago's stoppage-time strike confirmed a third consecutive league defeat on away soil for Eddie Howe's side.
After observing a moment of silence before kick-off on Remembrance Sunday, Newcastle created the opening goalscoring opportunity in front of goal as Bruno Guimarães collected Barnes' pass in a dangerous area but the Magpies skipper blazed a 25-yard effort over the crossbar.
Newcastle survived an almighty goalmouth scramble later as Mikel Damsgaard's whipped free-kick was pushed away by Nick Pope before the Magpies goalkeeper prevented Nathan Collins' follow-up inside a crowded box, with Dan Burn eventually clearing the ball from the penalty area.
After Joelinton was forced off before the half-hour mark, with the Brazilian midfielder unable to continue after an early clash with Ouattara, Newcastle took the lead in the 27th minute through Barnes' first Premier League goal of the 2025/26 season.
As Jacob Murphy's low cross into the box was cleverly flicked into Barnes' path by Nick Woltemade, the Burnley-born winger evaded the challenge of two Brentford players before slamming the ball through the legs of Bees goalkeeper Caoimhín Kelleher, achieving a half-century of English top-flight strikes in the process.
Newcastle maintained their one-goal cushion despite the West London outfit almost drawing level in swift fashion, with Pope punching Michael Kayode's long throw into Aaron Hickey's path before the Scottish defender's resulting strike was importantly blocked out for a corner by Malick Thiaw.
Pope produced a key stop to prevent Yehor Yarmolyuk from Brentford from drawing level in the opening minutes of the second-half as the England international blocked the Ukranian's low strike with his feet.
In the 56th minute, though, Newcastle found themselves back on level terms through Schade's leveller. After the Magpies were unable to clear Kayode's long throw into the penalty area, the ball landed kindly into the German's path who nodded the ball into the net from close-range.
United were fortunate not to concede a penalty after the hour mark as Burn, booked two minutes prior, looked to have made contact with Ouattara inside the box but following a review, VAR deemed minimal contact from the Blyth-born defender.
Pope produced a key stop to prevent Thiago from firing Brentford ahead as the Brazilian's volley from the edge of the box forced the United 'keeper into tipping the ball over his own crossbar.
In the 78th minute, Newcastle fell behind though Thiago's penalty. After Burn was adjudged for bringing down Ouattara inside the box, referee Stuart Attwell brandished a second yellow card to the Magpies defender while Aaron Ramsdale was also introduced to the field in place of the injured Pope but the 27-year-old 'keeper was unable to deny Thiago from 12 yards, who slotted the ball into the net.
With 11 minutes of stoppage-time awarded, Newcastle's hopes of a late equaliser were distinguished as Thiago netted Brentford's third goal of the afternoon in the dying stages. After Thiaw slid in to win the ball from Reiss Nelson, the ball fell kindly into Thiago's path who nipped in between Sven Botman and Ramsdale before slotting the ball beyond the latter, capping off a difficult afternoon in the capital for the visitors.
