Haiti international Dumornay also had opened the scoring six minutes earlier in the 36th to help lift Lyon to three straight wins and no goals conceded as it seeks a first Champions League title since 2022.
Chelsea coach Sonia Bompastor, who led Lyon to that Champions League title three seasons ago, now has a perfect record of nine wins in nine games in her debut season with her new club.
Celtic pushed Chelsea hard Wednesday, and the 2021 beaten finalist had to rally for a 2-1 win in Glasgow.
Chelsea stayed three points clear of Real Madrid, which was inspired by playmaker Caroline Weir who had two assists before curling in an elegant free kick to score herself in a 7-0 rout of Twente.
Wolfsburg forward Rebecka Blomqvist struck a second-half hat trick in a 5-0 win at Galatasaray to earn the two-time European champion its first points in the group led by Lyon.
Return games are played next Wednesday.
At the midway point of the group stage, all the leaders have three straight wins: Lyon, Chelsea, Bayern Munich and Manchester City, which opened last month by beating defending champion Barcelona.
Lyon's star
Dumornay was the competition's young player of the year last season, and her vision and smarts were evident in the 42nd when the ball was diverted to her by teammate Lindsey Horan, the United States captain, who intercepted a pass at the halfway line.
Dumornay took one touch then chipped the ball high in the air. It landed just over the goal line and bounced up into the net.
Canada international Vanessa Gilles sealed the win in the 52nd, pouncing on another loose ball in the goalmouth after a corner.
Chelsea wins again
Celtic's women rose to playing their first European game in the club's storied stadium at Parkhead and shocked Chelsea by taking a 22nd-minute lead with its first counterattack. Murphy Agnew ran clear to reach a long pass from Shannon McGregor and beat goalkeeper Zecira Musovic.
Chelsea leveled six minutes later when Japan international Maika Hamano finished a pass from Aggie Beever-Jones. It was 2-1 after a slick passing move in the 32nd, though it took a third shooting chance before Ashley Lawrence scored.
Beever-Jones was sent off in the last minute of stoppage time for a second yellow card.
Madrid's home comforts
Madrid followed up a 4-0 win over Celtic in its previous home game with another shutout inside the Alfredo Di Stéfano Stadium at the club's training ground.
The seven-goal rout started in the third minute when Weir’s shot aimed at the far post turned into a pass for Signe Bruun to score easily.
Defender María Méndez scored twice with headers from corners, one in each half, and France international Naomie Feller met a floated cross by Weir with a glancing header in the 50th.
Scotland’s Weir got her goal five minutes later, placing her 22-yard (meter) shot into the top corner of the net.
Right-back Oihane Hernandez broke clear to finish a move she started by breaking up Twente’s passing in midfield. Substitute Carla Camacho scored in stoppage time when the Dutch visitors could not clear yet another corner.
First points for Wolfsburg
In a meeting of two teams with zero points at kickoff, Wolfsburg dominated Champions League newcomer Galatasaray in Istanbul. Defender Marie Joelle Wedemeyer was unmarked at the far post to score with a header from a corner in the 24th.
Sweden forward Blomqvist scored with shots in the 63rd, 77th and in stoppage time. Later in added time Vivien Endemann completed the scoring. Wolfsburg ended the game with 33 goal attempts, 14 on target, compared to two for Galatasaray.
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