They lived in Pressburg (now Bratislava, Slovakia) and had two sons: Maximilian Karl Lamoral Graf O'Donnell von Tyrconnell, who saved the life of Emperor Franz Josef I of Austria in 1853; and Maurice Jr, Moritz O'Donnell.
In 1799 he was promoted to the rank of captain in the 54th Infantry Regiment, guarding the Wallachian-Illyrian frontier of the Empire.
In 1802 he took leave and made a voyage to Italy, where he met the renowned Madame de Staël during five days in Venice.
It was the beginning of a long relationship with Europe's leading socialite that lasted many years, with an intense correspondence with Madame de Staël, who at one time beseeched him to marry her.
On 11 April 1813, he was promoted to the rank of lieutenant-colonel, and took part in campaigns in France, where he distinguished himself at the siege of Besançon.