The Serie A debutants have lost all 13 games – breaking United’s 12 at the start of the 1930-31 season.
Alessandro Matri swept Sassuolo into the lead, but Samuel Armenteros equalised before Gaetano Letizia was sent off for the visitors.
Domenico Berardi missed a Sassuolo penalty before Federico Peluso won it.
Benevento, who won back-to-back promotions to reach Serie A, thought they were going to get their first point of the season when Berardi’s injury-time penalty hit the bar, but Peluso’s 94th-minute header consigned them to yet another defeat.
United were relegated from the First Division in 1930-31, winning only seven of 42 games after losing their opening 12.
