Following the successful Virginia campaign and the Battle of Yorktown, he returned to France and rejoined his unit in the Royal Army.
When the French Revolution began he was elected to the Estates-General and served in the subsequent National Constituent Assembly as a representative from Metz.
At the dissolution of the Assembly in 1791, he rejoined the army as a lieutenant general and the following year replaced Nicolas Luckner as commander-in-chief of the Army of the Vosges.
In 1792, he successfully led campaigns in the middle and upper Rhine regions, taking Speyer and Mainz and breaching the Wissembourg lines.
Following Charles François Dumouriez's apparent treason, the Committee of Public Safety investigated Custine, but a vigorous defense mounted by the Revolutionary lawyer Robespierre resulted in his acquittal.
Upon return to active command, he found the army had lost most of its officer corps and experienced troops, and in 1793, following a series of reversals in the spring, the French lost control of much of the territory they had acquired the year before.
Ordered to take command of the Army of the North, Custine sought first to solidify French control of the important crossings of the Rhine by Mainz.
Custine was found guilty of treason by a majority vote of the Tribunal on 27 August, and guillotined the following day.
His son was also executed a few months later, and his daughter-in-law suffered for several months in prison before she was released in the summer of 1794.
The fate of the family is representative of the fates of many of the minor aristocracy in France, especially those in the military and diplomatic corps, whose reputations the Montagnards tarnished in the Reign of Terror.