Monaco was runner-up behind PSG last season, but has been hampered by injuries in recent weeks and the hole left by Fabinho's departure to Liverpool during the offseason has not been filled. In the absence of the rock-solid defensive midfielder, center-backs Kamil Glick and Jemerson have been struggling.
After Djibril Sidibe could not stop Flavien Tait down the left flank in the 27th minute, Jemerson deflected the ball into Stephane Bahoken's path and the Angers forward found the net with a composed finish.
Monaco improved a bit after the interval but could not create dangerous chances, extending its winless run to seven matches in all competitions.
The result lifted Angers to ninth place in the standings, four points above Monaco.
NANTES LOSES AGAIN
Nantes also failed to shake up its poor form and lost 2-1 to Nice at home, remaining one point behind Monaco.
Patrick Vieira's side handed the eight-time champions their fourth defeat in seven matches, putting coach Miguel Cardoso under more pressure.
Cardoso's changes to his team did not pay off as his players lacked imagination and exposed themselves to counterattacks. Christophe Jallet put the visitors in front in the 31st minute from Allan Saint-Maximin's assist and Jean-Christophe Makengo scored a second-half winner after Emiliano Sala put the teams level.
SAINT-ETIENNE MOVES UP TO THIRD
Saint-Etienne continued its resurgence to provisionally climb into third place with a 3-2 win at Toulouse.
It was Saint-Etienne's second straight win after picking up just three points in its previous four games.
Saint-Etienne goalkeeper Stephane Ruffier made a decisive save in stoppage time to deny Ibrahim Sangare's header after Jimmy Durmaz pulled one back for Toulouse with a superb curled kick at the far post from the edge of the area.