His parents were Pierre Ernest de Mercy († 1619), governor of Longwy and Chamberlain of Duke Charles III of Lorraine, and Anne du Hautoy.

In 1625 he held the rank of captain in the regiment of Hannibal von Schauenburg and in 1626 he was mentioned as chamberlain of Archduke Leopold V of Tyrol.

For his brother Ludwig (born 1614), who died of his injuries at the age of 19 on 6 October 1633 shortly after the last attack by the Swedes, Franz von Mercy had an artistic bronze epitaph made in the Konstanz Minster, which is still preserved today.

[2] In 1636 the Imperial army stationed in Lorraine and Burgundy under Matthias Gallas planned an attack on Paris from the south, but was stopped by French troops under Bernard of Saxe-Weimar.

In November, Longueville entered the Rheingau but was soon pushed back by Mercy and Geleen who ended their campaign with regaining Alzey and Bacharach.

[7] The following year, Mercy took part at the campaign of the main Imperial army under Archduke Leopold Wilhelm and Ottavio Piccolomini against the Swedes under Johan Banér.

Repelling the Swedes and their allies to Hesse-Kassel, the Imperials and Bavarians laid in camp near Fritzlar for weeks in the immediate vicinity of their opponents.

When the thawing of the icy Danube stopped the Swedish advance, Imperial and Bavarian troops rallied and forced the Swedes to retreat in flight.

Continuing on the heels of the Swedes, Mercy had to hand over command to his superior Joachim Christian von Wahl at Zeitz on 1 May.

Only after Imperial and Bavarian forces had captured numerous towns in the surrounding countryside, their opponents abandoned the siege of Wolfenbüttel in September.

Later in 1645, fighting once more against Condé and Turenne, Mercy was killed by a musket ball at the Battle of Nördlingen (or Allerheim) on 3 August while leading reinforcements to the focus of the action.

[5] The French Marshal Gramont, who had been captured at the battle, reports of the exuberant reception that Mercy received from the people of the city who had hurried to the gates.