The France U-21 striker did not manage to find the net in any of his 14 appearances last season, having joined from Auxerre on a free transfer but seeing his progress hampered by a back problem.
As £30m summer signing Alex Sanchez watched from the bench, Sanogo produced a clinical display as he bagged a first-half hat-trick while Costa Rica forward Joel Campbell, back from a loan spell at Olympiacos, also impressed, scoring his first goal in an Arsenal shirt to suggest manager Arsene Wenger already has plenty of attacking options rather than needing to spend big again.
Wenger had named a mixed side, which included teenager Calum Chambers, the £16m signing from Southampton, lining up in defence along with Spanish youngster Hector Bellerin, while the likes of Sanchez, France full-back Mathieu Debuchy - purchased from Newcastle - and England midfielder Jack Wilshere all on the bench.
Arsenal's German FIFA World Cup™ winning trio Per Mertesacker, Lukas Podolski and Mesut Ozil are not scheduled to be involved until after the FA Community Shield against Manchester City at Wembley next weekend, with Crystal Palace the first opponents when the English Premier League returns on 16 August.
Benfica had made a bright start, with Nicolas Gaitan floating a shot onto the top of the crossbar, with reserve goalkeeper Damian Martinez beaten. Campbell spooned a volley wide from Ramsey's chipped through ball and then Sanogo stabbed a low cross from Bellerin past the near post.
Sanogo, however, did find his range on 26 minutes, flicking the ball past Moraes Artur after Ramsey had spun into the Benfica half and swept down the right flank. Benfica rallied briefly, but were 2-0 down on 40 minutes.
This time it was 19-year-old right-back Bellerin, on loan at Watford for a spell last season, who scampered away, before exchanging a neat one-two with Sanogo. He then picked out Campbell arriving at pace into the left side of the penalty area, and he drilled a fine left-foot volley into the bottom corner.
Arsenal extended their lead on the stroke of half-time, with Sanogo knocking in a pass from Campbell as the Benfica defence pushed up and called for offside. Sanogo completed his hat-trick in first-half stoppage time, hooking the ball home from Kieran Gibbs' left-wing cross.
The former Auxerre forward - whose career in France was interrupted by a broken leg - chalked up his fourth at the start of the second half, slotting past Artur after Ramsey's shot was saved.
Chile forward Sanchez, signed from Barcelona, was introduced to rapturous applause when he replaced Campbell with 15 minutes left.
In the earlier kick-off, Monaco twice came from behind to draw 2-2 against Valencia. Colombia striker Falcao, who missed the World Cup because of a serious knee injury, made an appearance off the bench for Monaco in the closing stages, his first since January.