From 1854 to 1857, Mosengel studied at the Kunstakademie Düsseldorf under Hans Fredrik Gude, and from 1858 to 1859 in Paris. In 1861, he studied in Geneva with Alexandre Calame, before settling in Hamburg.
He had built a reputation painting Alpine scenes, and later turned to scenes from Westphalia. In 1879 he traveled to the lakes of Northern Italy, where he painted en plein air. Mosengel and the Nietzsches were staying in the Hotel Alpenclub.
The Franco-Prussian War was about to break out, and Nietzsche had been considering joining the Prussian medical corporations
Apparently Mosengel "strengthened resolve to do so". Besides Leopold Rau, Mosengel is likely the only painter Nietzsche knew personally.