In 1789, over the age of fifty, he married Françoise Cathérine Kervyn de Oud Mooreghem (1744-1824), widow of Jean-Pierre Zoetaert, Secretary to the Raad van Vlaanderen, the highest court of the County of Flanders.
In 1760 Karel Jozef de Graeve became lawyer in Gent and for some time functioned as Pensionary to the city of Bruges.
A few days after his work on a manifesto on November 4, 1789 on behalf of the Flemish Court concerning the Austrian government the Patriots conquered Ghent and de Graeve was appointed member of the 'Comité civil' that was in charge of the town.
On November 5, 1789 he composed a conception for a manifesto, on behalf of the Court of Flanders, written in Alexandrines, that was meant to be sent to the Austrian government, seated in Brussels.
Meanwhile De Graeve finished a historical work, that was published postumely by his friends in 1806 in three volumes under the title: République des Champs Elysées, ou Monde ancien.