He became the founding director of the German entomological institute where he collaborated with entomologists around the world.

He is not to be confused with the American entomologist George Henry Horn who also studied Coleoptera.

In 1889 he met Gustav Kraatz who influenced his studies and in 1891 he published his first paper on tiger beetles along with his schoolmate Hans Roeschke.

Horn established a journal where he reviewed literature and interacted with colleagues across the world.

[1][2][3][4][5][6] Horn's collection of Cicindelidae including larvae as well as his other beetle collections (North Africa (1896), Ceylon (1899), North and South America (1902) and Persian Gulf (1926) are conserved in the German Entomological Institute.