
After a 0-0 draw with Lille was followed by a 2-1 derby defeat to Nice, Monaco were undone by a fine 29th-minute strike from Jean-Eudes Aholou.
The defeat leaves Monaco, who travel to Bologna in the Champions League on Tuesday, in danger of losing second place in the table to Marseille or Lille.
Champions Paris St-Germain still lead by three points and they could extend that advantage when they host Lens today.
In stark contrast to Monaco’s fortunes, Angers have now won two games in a row.
Their 4-2 win over St Etienne last week was their first of the season after they had drawn four and lost four of their opening eight games.