Les Verts showed their determination to make an impact with two shots early off target, but it was third time lucky for Saint-Etienne on 10 minutes, as Stassin headed Léo Pétrot’s cross through Remy Descamps’ legs for his 11th goal of the season.
St Etienne’s leading marksman was in bother just 13 minutes later, though, as referee Francois Letexier brandished a straight red card for a nasty-looking stamp on the back of Corentin Tolisso’s ankle, leading to the Lyon midfielder forcibly leaving the pitch on a stretcher.
But after consultation with the pitchside VAR monitor, Letexier decided to rescind his initial card and downgrade it to only a yellow.
The hosts continued to be the superior side as half-time approached, and Zuriko Davitashvili somehow failed to double the advantage.
Despite having only Descamps to beat, the Georgian striker badly misfired his effort wide when it looked easier to score or square it to a teammate to finish. But scenes then took an ugly turn off the pitch when a linesman was struck by an object thrown from the crowd, leading to a 30+ minute suspension in play.
Lyon manager Paulo Fonseca decided to switch it up during the break as he introduced the more attack-minded pairing of Ainsley Maitland-Niles and Thiago Almada.
Those changes prompted the visitors into immediate action, as Alexandre Lacazette firstly managed to skip around St Etienne goalkeeper Gautier Larsonneur but failed to find anyone in the area, before Duje Caleta-Car planted a header only inches wide.
At the opposite end, Davitashvili spurned another opportunity to put the game to bed for his relegation-threatened side as he fired straight at Descamps, before Pierre Ekwah then struck the underside of the crossbar with a breathtaking long-range strike.
However, Stassin succeeded where the Sunderland loanee failed at the three-quarter mark, crashing a stunning strike from distance past Descamps after being allowed to motor towards goal unchallenged.
It wasn’t quite party time yet, though, and Tanner Tessmann set up an intriguing final 14 minutes by halving the visitors’ deficit with a sublime side-footed finish.
Yet, despite having their backs pinned to the wall, St Etienne remained calm to secure what could turn out to be a pivotal three points in their fight against relegation.
Meanwhile, the defeat sees Lyon slip to sixth position as their mission to qualify for Europe next season hit a stumbling block.
Flashscore Man of the Match: Lucas Stassin (AS Saint-Etienne)