Arsenal came back from losing the first leg at home to beat the French giants 4-1 away in their semi-final tie last season before also stunning Barca in the final to become the first English winners of the competition for 18 years.
However, the Gunners have made a poor start to the Women's Super League season and suffered a damaging defeat to their hopes of a top-four finish that would secure direct passage to the Champions League quarter-finals.
Arsenal made the perfect start thanks to Alessia Russo's cool finish on seven minutes.
Lyon were gifted a route back into the game when Arsenal goalkeeper Daphne van Domselaar's mishit pass went straight to Dumornay, who fired in at the second attempt after her first effort was saved.
The Haiti international then pounced on more slack Arsenal defending to fire into the top corner for a second goal in five minutes midway through the first half.
Barcelona kicked off their quest to reclaim the trophy with a demolition of Bayern.
Pere Romeu said his team, who have reached five consecutive Champions League finals, would "compete like animals" and three goals in the first 27 minutes to set up an emphatic victory.
Alexia Putellas opened the scoring after just four minutes, curling into the top corner from the edge of the box.
Three-time winners Barca extended their lead through Ewa Pajor and Esmee Brugts, before Klara Buhl pulled one back for the beleaguered visitors.
Salma Paralluelo restored Barca's three-goal advantage before half-time, and Pajor netted again after the break.
Claudia Pina capped the victory with a late double and with recently crowned Ballon d'Or winner Aitana Bonmati pulling the strings in midfield, Barca showed why they remain favourites for the competition despite last year's disappointment.
Juventus beat Benfica 2-1 thanks to Cecilia Salvai's double, while Paris FC were held 2-2 by Leuven in the day's other matches.
English champions Chelsea travel to FC Twente and Wolfsburg host Paris Saint-Germain in the pick of Wednesday's action.
For the first time this season, the women's Champions League has 18 participating teams, playing six games each against different opponents in a league phase.
Only the top four qualify directly for the quarter-finals, with sides ranked fifth to 12th involved in a play-off to join them in the last eight.