He planned to open a piano store in the following years, but the project failed for lack of funds. He moved to Milan in 1805 and studied music with Bonifazio Asioli, though he gave up his studies in 1810 to become an official in the service of the Austrian financial administration and the governmental accounting department in Milan. He also served as official translator for Italian for the Austrian Court Chamber. He owned a house in the village of Caversaccio in Valmorea, Province of Como; he appreciated the amenities of the place and the wholesomeness of the water. He bequeathed the house to the town, which is stated on a plaque dedicated to him. The Town Hall keeps a copy of the will.
He also frequently attended events related to his father until his death in Milan in 1858. He neither married nor had children.
^Erich Schenk (1960). Mozart and His Times. Secker & Warburg. p. 450. "Before he died on October 31, 1858, as a pensioned-off public accountant in Milan, Karl Thomas was the recipient of the benefits of legislation ... From royalties on three Paris performances of Figaro he received some ten thousand francs, enough to buy a country estate at Caversaccio in Como Province."