City confirmed their place as Group F winners in Europe's top competition with a 2-1 win over Hoffenheim, despite going 1-0 down at Etihad Stadium.
Sergio Aguero was again missing with a muscular injury leaving Sane to put in a star performance, netting in the dying moments of the first period before snatching a winner in the 61st minute.
Speaking to Manchester City's official website following the game, Sane said: "After training I practise a lot and try to score some free-kicks. Today I just thought we had this moment, and I just tried it."
When asked if it was the best free-kick he had ever scored, the former Schalke man laughed: "Yes, I think in my professional career I've never scored a free-kick, that was the first one, so it was the best."
Lyon finished as runners-up to City as they only managed a draw away to Shakhtar Donetsk, meaning the Manchester club comfortably topped the table by five points despite Bernardo Silva missing a simple chance in the second half after being set up by Sane.
The Citizens' position as group leaders looked precarious when they went behind in the 16th minute, with Lyon only needing to win their match in order to overtake Pep Guardiola's side at that stage.
Sane led the fight-back though, and said his team did well after getting themselves into a difficult situation.
"We had some good moments, we made life hard for ourselves but it was a really good game," the forward continued."I was thinking of whether to shoot or not from the free-kick, it was quite a distance. It was quite lucky.
"We had enough chances in the second half to make the game quiet, I don't know what happened with the three-on-one, I passed it to Bernardo Silva and he had a tap-in...I wanted an assist!"
City will now face a group runner-up when the Champions League round of 16 draw is made on Monday.
"We look forward to the draw, we will watch it together and we will see who we will face next," Sane added.