Monaco makes it five straight wins
PSG bounced back from a poor run of form by defeating Rennes 3-1 to stay two points clear of Monaco.
Anthony Martial, one of the most promising prospects in the league at 18, gave Monaco the lead after three minutes with an assist from Colombia playmaker James Rodriguez.
Guingamp was down to 10 men from the 22nd when defender Jonathan Martins Pereira picked up a second yellow card.
Monaco capitalized on its numerical superiority as leftback Layvin Kurzawa doubled the lead by heading home a corner in the 42nd.
Monaco striker Radamel Falcao missed a fourth straight match because of a thigh injury.
Also, Toulouse beat Nantes 2-1, Reims scored in stoppage time to salvage a 1-1 draw with Evian, Lorient won a fifth straight match beating Ajaccio 2-1, and Nice ended a seven-game losing streak edging Sochaux 1-0.