Fredderic Hantz's men came into the match with dreams of easing their relegation worries but it was compounded despite the Senegal international's effort.
Jorge Marco de Oliveria Moraes got the hosts' opening goal just before the halftime break.
On return from recess, Metz suffered a huge blow when goalkeeper Eiji Kawashima got sent off in the 50th minute, forcing lively Jonathan Rivierez off for Quentin Beunardeau.
In the 67th minute, Monaco capitalized on their numerical advantage to increase the tally through Rachid Ghezzal after his superb curling shot breezed past Beunardeau.
Hantz's men pulled one back through substitute Niane who headed home Farid Boulaya's cross in the 72nd minute but Rony Lopez banished hopes of a comeback for the 10-man visiting side when he converted a Fabinho pass to cement victory for the Red and Whites.
Niane's goal was his first for Metz in his third league appearances this season, while Ghezzal registered his second in 15 games.