The Brazilian has endured an injury-plagued season, but was on hand to net his side’s second with a finish into an unguarded goal after Aaron Ramsdale had pushed out Nicolas Pepe’s shot.
It was the 19-year-old’s first goal since January 2020.
It came between two Alexandre Lacazette goals – the first a neat first-half finish after Dani Ceballos’ back-heel and the second a clinical effort in the closing stages after he was put clear by Thomas Partey.
Sheffield United have won only four games this season and none since Paul Heckingbottom took caretaker charge a month ago and they had little answer to their opponents’ increased efficiency.
At 1-0 there was always a chance that this generous Arsenal side may provide the Blades with a way back in, but Martinelli’s strike ended such home hopes.
The win lifts Arsenal above Leeds and Aston Villa and up to ninth in the table, with 45 points.
Sheffield United remain rooted to the bottom, 18 points from safety with just 21 left to play for. With their woeful goal difference, a loss at Wolves next Saturday will all but confirm their relegation.