Arsene Wenger's side are four-time champions and the current holders of the pre-season tournament, but they had to come from behind at a wet Emirates Stadium with Sanchez still absent following his Confederations Cup participation with Chile.
Franco Cervi put Benfica ahead in the 11th minute, the Argentine's strike taking a crucial double deflection off Arsenal captain Per Mertesacker.
Persistence from Sead Kolasinac soon provided an equaliser for the hosts, however, the new signing robbing Pizzi and chipping a cross for Theo Walcott to volley home.
Walcott added another after the half-hour mark, slotting in from close range after Kolasinac and Francis Coquelin combined on the left side of the box, but Benfica levelled before the interval when Eduardo Salvio beat David Ospina with the aid of another deflection.
But Arsenal showed their superiority after the interval, Lisandro Lopez turned Walcott's cross into the back of his own net before Olivier Giroud got on the scoresheet and teed up Alex Iwobi for the best goal of the game to complete the scoring.
Julio Cesar made a point-blank save to deny Giroud in the fourth minute, but it was Benfica who scored the opening goal.
The visitors crafted a fine attacking move and a big deflection off Mertesacker saw Ospina beaten by Cervi's low effort.
Walcott volleyed in the equaliser in the 24th minute, the winger on hand to finish off Kolasinac's cross after the left wing-back regained possession in the Benfica box.
And super Arsenal build-up play soon provided Walcott with a second, Kolasinac sending Coquelin clear to square a low cross for him to tap home.
Ospina parried Pizzi's shot but the goalkeeper could do little about Benfica's equaliser as the goals continued to flow in the first period.
Arsenal gave the ball away in their own half and Benfica worked the opportunity well, Salvio's effort wrong-footing the Colombia international thanks to a nick off Rob Holding's.
Another deflection saw Arsenal retake the lead seven minutes after the interval, Lopez diverting Walcott's delivery into his own goal with Giroud lurking behind him.
Walcott blazed a chance to seal his hat-trick over, but Giroud extended the lead in the 64th minute when he steered in a fine cross from right wing-back Reiss Nelson.
Kolasinac and Giroud set up Iwobi for a thumping finish and there was still time for Arsenal's record signing Alexandre Lacazette to make his home debut from the bench.
Arsenal take eight points from the game - as Emirates Cup rules award teams a point for every goal - and they will face Sevilla on Sunday in the tournament's final match, while Benfica take on Bundesliga side RB Leipzig.