In 1791, he spent a year as a novice in the Teutonic Order in Bonn, but at the outbreak of hostilities between France and Austria in 1793, Mervelt rejoined the Habsburg army at as adjudant to the Prince of Saxe-Coburg.
In 1791, he spent a year as a novice in the Teutonic Order in Bonn. At the outbreak of hostilities between France and Austria in 1793, now a major, Mervelt fought at the Battle of Neerwinden and was Flügel (wing) Adjutant to the Prince of Saxe-Coburg.