England forward Saka capped a neat move to put Arsenal ahead, before his replacement Martinelli killed the Premier League contest with just his second touch.
Saka’s second Premier League goal of the season ensured Mikel Arteta’s men would hold onto fifth place in the table.
The 20-year-old hobbled off with a potentially minor knee knock, giving Martinelli the chance to round off the Gunners’ comfortable victory.
The Brazilian forward’s first Arsenal goal since May lit up a frosty north London, pairing accuracy with flair to delight the home hordes.
Newcastle’s winless league run extended to 13 games in an already desperate campaign, with not even new boss Eddie Howe’s touchline presence enough to turn the tide.
Former Bournemouth boss Howe was in the dugout having finally tested negative after previously contracting Covid-19.
Newcastle kept their shape manfully in the first half with Jonjo Shelvey even striking the woodwork from distance, but the St James’ Park men were well beaten in the end.
Callum Wilson failed to make the most of an early Newcastle break, before Saka nudged Albert Sambi Lokonga’s angled ball wide at full stretch.
Martin Odegaard saw a well-whipped free-kick smartly saved by Martin Dubravka as Arsenal started to take control.
Shelvey unleashed a piledriver strike to cut the Gunners’ one-way traffic, only to see Aaron Ramsdale tip the ball onto the bar.
Shelvey could not repeat the feat with another long-range strike moments later, but the hosts escaped danger despite Ramsdale spilling what should have proved a routine stop.
Smith Rowe and Aubameyang then conspired for the double miss of the half, spurning gilt-edged openings to put the home side ahead.
Saka’s byline cross begged to be buried but Smith Rowe nodded into the middle of the goal, allowing Dubravka to parry.
The lurking Aubameyang’s eyes widened with intent, only to see his side-footed effort graze the outside of the post from a yard off the line.
Fabian Schar’s astute intervention thwarted Saka raiding down the inside right in the Newcastle box, and then Smith Rowe saw a goalbound shot deflected wide.
Newcastle reached the half-time interval far the happier with the game goalless then, but quickly saw their outlook turn bleak after the break.
Arsenal finally exploited the space in Newcastle’s inside forward channels, and duly scored twice.
First Smith Rowe fed Nuno Tavares who in turn flicked around the corner to Saka.
The Gunners carved Newcastle open with a simple third-man running routine, and Saka slotted home with ease to swipe the lead.
No sooner had the England forward trudged off with a minor injury issue though, than his replacement doubled the home advantage.
Martinelli latched onto Takehiro Tomiyasu’s chipped pass into the inside-right channel, delivering a first-time finish of genuine quality.