He was part of the neo-Friesian school (named after post-Kantian philosopher Jakob Friedrich Fries) of neo-Kantianism and a friend of the mathematician David Hilbert.

From 1903 to 1904, he worked with mathematicians and philosophers at the University of Göttingen, such as his doctoral advisor Julius Baumann, David Hilbert, Felix Klein, Carl Runge, and his later rival Edmund Husserl.

It sets out to find a "critique" on science and metaphysics, similar to empiricism,[8] as things can only be true based on the perceptions and limitations on human minds.

Nelson continued defending Fries' philosophy and ideas by publishing a neue Folge (new series) of Abhandlungen der Fries'schen Schule (1904) with Gerhard Hessenberg and mathematician Karl Kaiser.

[13] They started working with an education center called Landerziehungsheim Walkemühle, founded in 1921 by a support of Nelson, progressive teacher Ludwig Wunder (1878–1949).

As a result, in 1925, he and Minna Specht founded the Internationaler Sozialistischer Kampfbund (ISK; "International Socialist Militant League") merging it with the IJB by taking over its publishing label, Öffentliches Leben.

[10] Among Leonard Nelson's students and political companions in the International Socialist Kampfbund were also[15] Prime Minister Alfred Kubel (1909–1999) and journalist Fritz Eberhard (1896–1982), later member of the Parlamentarischer Rat.

[19] Nelson was an insomniac and died at a young age from pneumonia, and was buried at a Jewish cemetery in Melsungen alongside his father Heinrich.

[20] Nelson's ideas continued to have an impact upon German socialism and communism in Nazi Germany as the ISK's members became active in the left-wing resistance to Nazism.