Most of his pictures are large-format and depict scenes in Norway and the Alps. He was a pupil of Johann Wilhelm Schirmer, a landscape painter of the Düsseldorf school. Leu travelled in Norway in 1843 and 1847, and later travelled widely in the Alps.
His Norwegian paintings raised awareness in Germany of that country"s scenery.
He lived for a time in Brussels, then returned to Düsseldorf. In 1855 he received an honourable mention at the Paris Exposition.
Leu died in Seelisberg, Switzerland.
In 1882 he moved to Berlin, where he became a royal professor and a member of the Academy of Artist He was also a member of the Vienna, Amsterdam and Brussels Academies.