Twenty-year-old Brazilian star Martinelli netted a first-half brace which together with Bukayo Saka's strike put the dominant Gunners 3-0 up at the break.
Leeds pulled a goal back in the 75th minute through a Raphinha penalty which was awarded after Ben White fouled Joe Gelhardt but Mikel Arteta's side bagged a fourth goal with four minutes left through substitute Emile Smith Rowe.
"Today we were good away from home," Martinelli told BBC Sport.
"I think we did our job and we played so well. It was a great performance.
"I was trying to score more - but the most important thing is the victory."
Speaking to Sky Sports, Martinelli said: "It was a very good result. It’s so important for the team. All the team played well and we’re so happy with the work and the result.
"That’s what Mikel asks from us. Every time we go into the final third we need to be dangerous."
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Thank you Laura Collins