Rominigue Kouame fired in from the edge of the area on the half-hour for Troyes, but Ben Yedder's quick-fire double in the final 10 minutes seemed to have won the game for Monaco.
Substitute Ike Ugbo snapped Troyes' five-match losing streak though with a header in the first minute of added time.
Monaco, who were knocked out of the Europa League by Bayer Leverkusen last month, missed the chance to go second but did reclaim third place from Lens on goals scored.
Philippe Clement's side are a point behind Marseille, who visit Rennes later on Sunday and occupy the other automatic Champions League spot behind Paris Saint-Germain.
Troyes remain in the relegation zone and have picked up just two points from their past nine matches.
PSG are 11 points clear at the top after Kylian Mbappe became the club's all-time top scorer with his 201st goal in a 4-2 win over Nantes on Saturday.
Reims made it 18 league games without defeat as Jens Cajuste scored a last-gasp winner in a 1-0 victory over Ajaccio.
They moved up to eighth -- above Lorient and Lyon who played out a goalless draw, while Clermont won 1-0 at Toulouse.
Montpellier thrashed bottom club Angers 5-0 and Brest climbed out of the relegation places after beating Strasbourg 1-0.
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