Njie opened the scoring midway through the first half before Lindsay Rose was sent off in the 28th minute, allowing St Etienne to take the lead with a Max-Alain Gradel penalty and Romain Hamouma strike just before the break.
Jallet levelled three minutes into the second half to put Lyon on 65 points from 33 games, ahead of Paris St Germain on goal difference.
French champions PSG, who beat Nice 3-1 on Saturday, have a game in hand.
Les Verts are fifth on 57 points with a shot at a Champions League spot since third-placed Monaco, held to a 1-1 draw by Stade Rennais on Saturday, are just two points above.
At Gerland, St Etienne coach Christophe Galtier fielded a five-man defence but Lyon tore it to pieces as Njie slalomed into the area, dribbling past keeper Stephane Ruffier, only for Loic Perrin to save on his goal line.
In the 16th minute, Nabil Fekir's fierce strike smashed the bar as Lyon increased the pressure.
Eight minutes later, Lyon went ahead when Njie fired a low shot past Ruffier after being set up on the right side of the box by Fekir.
The hosts were rewarded in the 31st after Rose deflected Landry Nguemo's shot with his hand. The defender was shown a straight red car and Gradel converted the resulting penalty.
Lyon coach Hubert Fournier left his attacking trio of Njie, Fekir and Alexandre Lacazette on the pitch as he had no intention of sharing the spoils.
St Etienne, however, took the lead on the stroke of halftime when Hamouma went around Anthony Lopes after being set up by Benjamin Corgnet and pushed the ball into the empty net.
Lyon, however, continued to push hard and equalised three minutes into the second half when Jallet headed home from Fekir's free kick.
In the 71st minute, Perrin deflected a Lyon corner kick onto his own bar as the hosts tried their best to take all three points despite being one man down.
(Reporting by Julien Pretot, editing by Pritha Sarkar)
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