Mead and Russo made their contributions in the final 33 minutes in Lisbon, after the Gunners had lost 2-1 at home to Lyon last week.
Benfica are now in 15th place in the 18-team table having suffered a second straight defeat.
Elsewhere, Paris Saint-Germain are also winless after two games, having lost 2-1 in the French capital to Real Madrid.
The major drama came in Bavaria as Germany forward Lea Schueller netted after 95 minutes in Bayern Munich's 2-1 win over Juventus.
Earlier, experienced Sweden winger Fridolina Rolfo scored as Manchester United edged Atletico Madrid 1-0 to maintain their perfect start to the season
Atleti are ninth in the 18-team league phase, having hammered St Poelten 6-0 eight days ago.
Both sides finished with 10 players in the Spanish capital after United's Netherlands defender Dominique Janssen and the hosts' Spanish midfielder Alexia Fernandez were shown red cards.
Janssen was sent off shortly before half-time, while Atleti's comeback efforts were hampered with a quarter of an hour to go as Fernandez was dismissed and their hopes of even a point were dashed.