Leeds fielded a bare-bones side on Saturday with eight senior players missing through injury or suspension, and even named 15-year-old Archie Gray on the bench, who would have become the youngest player in Premier League history had he been substituted on.
Arsenal notched 11 shots on target in the first half in a dominant performance, with the first coming inside the opening five minutes as goalkeeper Illan Meslier pushed aside a shot from opposing striker Alexandre Lacazette before saving again shortly after, this time at his near post from a strike from midfielder Thomas Partey.
Martinelli broke the deadlock on 16 minutes as he ran onto the ball following a tackle from Lacazette and fired into the top right corner.
The 20-year-old doubled Arsenal's lead in superb fashion 13 minutes later when he chipped Meslier after baring down on goal for a one-on-one.
Saka added a third just before the break, standing up two Leeds defenders inside the 18-yard box before scoring after a deflection.
Arsenal continued to dominate in the second half, although could have been reduced to 10 men when the Video Assistant Referee reviewed a tackle from midfielder Granit Xhaka on Leeds midfielder Raphinha for a potential red card early in the second half, but no punishment was given.
Leeds netted a consolation goal on 75 minutes through a belting Raphinha penalty after striker Joe Gelhardt was taken down inside the box.
Smith Rowe capped his side's performance with a goal on 83 minutes, firing home a superb pass from teammate Martin Odegaard.
After the match, Arteta heaped praise on Martinelli, who has now scored four goals in his last six league games.
"We see the player every day and we know where the player has to be to be able to play at the top level all season," Arteta told reporters.
"We are here to develop him as a player and he has had to wait for his moment.
"I am really happy with him and how he is performing. He has come a long, long, way. His energy and his passion do not get much better. When he has the chances he puts them away. He is heading in the right direction."
Arteta confirmed after the match that Arsenal players sitting on the bench were racially abused by a Leeds supporter.
I can confirm it and it was reported to stadium manager," Arteta said.
"The authorities will investigate that. One of the players on the bench heard those comments. I don't know exactly who it was directed at."
Information from Reuters contributed to this report.