Inheriting a large fortune after the death of his father he went with Tieck and the brothers Riepenhausen on a study trip to Rome (1805–1806) where he met the group of German artists (among others, Joseph Anton Koch) living and working there and acquired the foundations of his expertise of the classical arts.
He made his first important contribution to Art history with the publication in 1812 of: Über die antike Gruppe Castor und Pollux ….
He met the German painters of the Nazarene movement who had been living and working in Rome since 1810 and became an important patron, supporting them through publications and purchases until he broke with the younger artists around Johann David Passavant and Julius Schnorr.
In 1822 Rumohr published under the name of his personal chef a treatise about the culinary arts, the Geist der Kochkunst von Joseph König., in praise and defence of traditional national (not just German) and provincial cuisine.
On his third Italian journey (1828–1829), accompanied by Nerly (who travelled further on to Rome), Rumohr negotiated purchases for the Berliner Gemäldegalerie, worked on a novel and acted as guide to the Prussian crown prince in Florence and Siena.
In the year 1834 he stayed in Copenhagen to catalogue together with Just Mathias Thiele the royal print collection; he was rewarded with the title of chamberlain and became patron to the young Danish painter Lorenz Frølich.
In spring of 1837 Rumohr went on his fourth Italian journey that led him to Milan and the surrounding region of Lombardy to study the traditional irrigation systems.