It rained goals in North London, but there were also a handful of defensive errors as both teams shape themselves up for the start of their domestic seasons.
Walcott, whose long-term future at Arsenal has been called into doubt, showed intent early on as he delivered a teasing cross into the box for Olivier Giroud, but his header from close range was stopped by goalkeeper Julio Cesar.
Arsene Wenger's men, who played the majority of their attacks in the first half down the right-hand side, managed to recover thanks to Walcott, who nabbed his second goal of the afternoon just after the half-hour mark.
The rebound dropped to Eduardo Salvio and the midfielder nestled the ball into the net after his shot deflected off Rob Holding, but Benfica could not add to their lead despite showing threatening signs early in the second half.
In fact, they were left red faced when Walcott managed to get past the defensive line and deliver a cross into the danger area, prompting Lisandro Lopez to slide in, but he could only send the ball into his own net.
After being gifted the lead, Arsenal ramped up their attack, and Walcott would have scored a hat-trick had he not blasted the ball over the crossbar after latching on to Giroud's pass.
The Frenchman, who has been linked with a move away from the Emirates this summer, gave Wenger something to think about when he set up Alex Iwobi's goal and Arsenal's fifth.
Kolasinac was involved as well as he fed through to Giroud, who showed strength in holding the ball up and knocking it into Iwobi's path for the Nigerian to score.