He received his diploma from the Akademie in 1896, having studied under the Norwegian genre painter Carl Frithjof Smith (1859–1917) and Max Thedy (1858–1924), then as a master’s student with Professor Theodor Hagen, considered one of the founders of German Impressionism.

In 1905, Diehl volunteered for his obligatory military service by joining a Hessian Infantry Regiment, spending one year in Darmstadt.

He sketched constantly and seemed most confident in creating colored landscape drawings, which demonstrate excellent perspectival rendering and shading.

This artist's organization was committed to a traditional idea of painting, emulating the Austrian-Swiss painter of symbolic Alpine landscapes, Giovanni Segantini (1858–1899).

Critics speak of his work as showing an individual streak within traditional painterly parameters—of going his "own way" [4]—and by the 1920s, Diehl was indeed formulating his own stylistic preferences.

In one of his most cryptic and intriguing works, Affenballade (Ballad of the Apes), he created a grid of bars that are painted along the lines of the Hessian sacks he used as a canvas.

[6] In the late 1920s, he began to create, apparently for private amusement, a series of eccentric, caricature-like watercolors that reveal a knowledge of works by such modern figures as Emil Nolde and Alfred Kubin.

[8] These sometimes anguished images are the pieces that have endured and are of greatest interest to contemporary audiences, who see in them a psychological reflection of the time between the wars in Vienna.

"[9] Hanns Diehl was, like so many of his contemporaries, a romantic nationalist—a German who had become a naturalized Austrian citizen in 1926, at a time when notions of national identity were particularly urgent, after the defeat in World War I and the fear of Soviet-style Communism engulfing the countries.

He became one of the directors of the Gemeinschaft bildender Künstler, the Society of Visual Artists, and submitted designs for the Party's official publications and events.

Diehl never expressed any anti-Semitic sentiment, and after the War, when he was briefly imprisoned, it was the testimony of a Jewish neighbor that assured his release.

The real reasons for his arrest in 1946 have been muddied, but appear to have more to do with a squabble over studio space that caused Diehl to be turned in to the authorities as a Nazi party member.

In 1963, his family organized an exhibition of his work at the Österreichische Staatsdruckerei, at which time the art critic of the Wiener Zeitung wrote that he was a painter who had been unfairly forgotten.