United made it four wins from four in the Champions League to all but seal qualification to the knockout stages.
The Reds require a solitary point from the two remaining Group A fixtures after Mile Svilar's luckless own goal and Daley Blind's late penalty maintained our 100% record.
CSKA Moscow won 2-1 away to Basel to boost their own hopes of qualification and force United to wait a little longer in the process.
This was far from a vintage United showing, as a much improved Benfica forced David de Gea into two world-class stops, one in each half.
Anthony Martial was rewarded for his decisive impact against Spurs with a starting place, but his evening did not get off to the best of starts.
In the 15th minute, the Frenchman went down under a challenge from Douglas, who handled the ball and referee Gediminas Mazeika pointed to the spot.
Unfortunately, Martial's resultant penalty was a poor one and 18-year-old Svilar pulled off a comfortable save.
David de Gea was soon called into action in spectacular style at the other end, when he tipped away Diogo Goncalves dipping effort from distance.
Romelu Lukaku was denied by Svilar who saved well from close range, but the keeper - whose error proved so costly in the reverse fixture - saw luck desert him again on the stroke of half-time.
Nemanja Matic collected the ball in space and thumped his shot goalwards from 25 yards, which struck the post and ricocheted in, off Svilar's back.
The Reds started the second half strongly and went close through Martial, who fired wide after neat link-up with Lukaku.
Benfica continued to pose a threat and went close twice in quick succession on the hour mark.
First, de Gea thwarted Goncalves with an excellent low save, and then Raul Jimenez clipped his shot against the post after Eric Bailly's uncharacteristic error.
Marcus Rashford, on his 20th birthday, was introduced in place of man of the match Martial and made an immediate impact three minutes after coming on.
After a mazy run into the Eagles penalty area, he earned United a second penalty of the match on 78 minutes.
Despite discussions between Ander Herrera and Lukaku, Blind took responsibility for the kick and despatched it with ease to put the result beyond any lingering doubt.
On Hallowe'en night, there were no late scares for United, who avoided a horror show and travel to Basel on Matchday Five needing one point to progress.
Overall team performance: 6.5/10
United Faithful Man of the Match: Anthony Martial. Provided a threat down the left with his pace and trickery.