Stunning goals from Gauthier Hein and Marvin Gakpa helped the Ligue 2 leaders to a 3-1 win at a sparsely populated Stade Louis II on Tuesday.
Hein opened the scoring in the 32nd minute with a curling effort, but Metz's lead was short-lived, with Monaco captain Radamel Falcao levelling by converting a Djibril Sidibe cross.
Another high-class finish put Metz back in front in the 62nd minute, though, Gakpa arrowing a 25-yard drive across goalkeeper Danijel Subasic.
Henry sent on Cesc Fabregas, a January signing from Chelsea, in an attempt to get back into the game. But the scoreline got even worse for Monaco in the 74th minute, Hein involved as Ibrahima Niane slotted in from close range.
And Frederic Antonetti's men could have added to their lead further only for referee Antony Gautier to harshly rule out a fourth Metz goal for a foul on Subasic.
A chastening defeat for Monaco comes after they were hammered 5-1 at home by Strasbourg in Ligue 1 on Saturday, a loss that saw former Arsenal and France striker Henry lose his temper.
Confronting defender Kenny Lala over perceived time-wasting, Henry appeared to use a French insult which translates as "grandson of a b****" or "your grandmother is a w****".
Henry has endured a difficult time at the principality since succeeding Leonardo Jardim in October.
The 41-year-old, in his first managerial position, has lost 11 and won just four of his 20 games in charge in all competitions, with just two of those victories coming in the league.
That has left the club in deep trouble at the foot of Ligue 1, just one point and one place off the bottom of the table.
They are still in the Coupe de la Ligue, however, with Monaco set to face Paris Saint-Germain's conquerors Guingamp in the semi-finals next week.
Next up in the league is a crucial trip to Dijon – who are just one place and two points above Henry’s side – on Saturday.
