Saka swept home ten minutes after the break to give Mikel Arteta’s side the lead against the Magpies, who had new head coach Eddie Howe in the dugout for the first time.
Substitute Martinelli made it two just after United had a strong appeal for a penalty turned down by referee Stuart Attwell, with Callum Wilson appearing to have been bundled over by Nuno Tavares.
The scoreline was harsh on Howe’s charges, who had shown plenty of resilience and endeavour before being undone by two moments of quality.
The Gunners did much of the early probing against a solid-looking United side, who started with four at the back. Albert Sambi Lokonga lifted a pass over for Saka to clip wide before Martin Dúbravka – handed a first start of the season by Howe – produced a smart stop to turn away Martin Ødegaard’s curling free kick.
Then it was Newcastle’s turn to threaten, and the willing running of Wilson and Joelinton helped them do just that. Fabian Schär glanced a Ryan Fraser corner over the crossbar before a swift counter resulted in Jonjo Shelvey trying his luck from 25 yards. It looked bound for the top right-hand corner, until Arsenal stopper Aaron Ramsdale produced a fine save to tip it onto the crossbar.
That was as close as United came in the opening 45, but the best chance of the half fell to the hosts. Emile Smith Rowe looked certain to score when Tavares chipped a cross into the six-yard box, but the England international’s header was kept out by Dúbravka. The Magpies’ number one parried the effort but only as far as Pierre-Emerick Aubameyang who somehow failed to find the net, directed his shot onto the post.
After the restart, Arteta’s men looked lively, with Saka’s trickery earning him some space in the box. His low shot was comfortably held by Dúbravka, though, who had been enjoying an impressive afternoon after his recall in place of Karl Darlow.
However, despite having made a number of earlier saves, the Slovakian was eventually beaten with 55 minutes gone. It was a neat move from the Gunners, involving Smith Rowe and Tavares, with the latter’s cute pass giving Saka a sight of goal. His low finish beyond Dúbravka was struck cleanly, leaving United with work to do.
Thomas Partey saw a strike deflected over the top as Arsenal sought to increase their advantage but at the other end, Newcastle saw a penalty appeal waved away. Shelvey’s magnificent through ball released Wilson, who looked to have been held back by Tavares and he tried to latch on to the pass and get a shot away.
But referee Attwell waved away the appeals, and just moments later Newcastle found themselves two goals behind. Takehiro Tomiyasu clipped a pass over the United backline towards Martinelli, who had replaced the injured Saka. The Brazilian took it first time, volleying it home in composed and clinical fashion.
Howe’s side responded, with Shelvey testing Ramsdale and Miguel Almirón adding some energy and urgency after his introduction, but ultimately they couldn’t haul themselves back into the contest.
Counter-attacks at both ends in the closing ten minutes brought half-chances – but no goals – for either Alexandre Lacazette and former Gunner Joe Willock, leaving Newcastle still searching for that elusive first win of the season.
