The Gunners, who were unbeaten in their previous 11 games untill RC Lens vs Arsenal, took the lead in northern France when Gabriel Jesus steered home a fine early finish – but they were pegged back by Adrien Thomasson's equaliser after David Raya's poor pass out the back.
Arsenal were then given a major injury blow when Bukayo Saka limped off just five days before a mouth-watering Premier League clash with Manchester City this Sunday.
And the Gunners, who looked off-colour, failed to generate too many clear-cut chances and were made to pay when Sepe Wahi steered home the winner for Lens with just over 20 minutes to go.
Kai Havertz, Leandro Trossard, Reiss Nelson and Emile Smith Rowe came close to adding to Arsenal's account, as the Gunners dropped to second in the group, one point ahead of Sevilla after their draw at PSV Eindhoven.
Havertz carved out an opening for Trossard early in the second half RC Lens vs Arsenal. He found him near the byline and the Belgian sat his defender down with a deft turn, but his tame shot didn't trouble Samba.
Then came one chance for each side in quick succession midway through the half – one taken, one not. First Odegaard's corner beat everyone in the box, and landed on the boot of the arriving Tomiyasu, but his fiercely struck first-time effort was batted away by Samba. Either side of the keeper and we would have been ahead.
As it was, Lens went up the other end and took the lead with another crisp finish. Przemyslaw Frankowski got away in the right wing, pulled his cross back for Wahi, who smashed home.
Mikel Arteta responded by sending on Emile Smith Rowe, Reiss Nelson, Ben White and Eddie Nketiah,
Smith Rowe had a 20-yard drive parried and Nelson had an effort cleared off the line, but we couldn't force the equaliser.