He was a prolific writer, churning out novels, poetry, plays, and essays. He was widely admired in the Netherlands in his own time for his writings, as well as his status as the first internationally prominent Dutch psychiatrist.
Van Eeden's psychiatrist practice included treating his fellow Tachtiger Willem Kloos as a patient starting in 1888. His treatment of Kloos was of limited benefit, as Kloos deteriorated into alcoholism and increasing symptoms of mental illness.
Van Eeden also incorporated his psychiatric insights into his later writings, such as in a deeply psychological novel called "Van de koele meren des doods" ("The cool lakes of death"). Published in 1900, the novel intimately traced the struggle of a woman addicted to morphine as she deteriorated physically and mentally.
His best known written work, "De Kleine Johannes" ("Little John"), which first appeared in the premiere issue of De Nieuwe Gids, was a fantastical adventure of an everyman who grows up to face the harsh realities of the world around him and the emptiness of hopes for a better afterlife, but ultimately finding meaning in serving the good of those around him. This ethic is memorialized in the line "Waar de mensheid is, en haar weedom, daar is mijn weg." ("Where mankind is, and her woe, there is my path.")
Van Eeden sought not only to write about, but also to practice, such an ethic. He established a communal cooperative called Walden, taking inspiration from Thoreau, in Bussum, North Holland, where the residents tried to produce as much of their needs as they could themselves and to share everything in common, and where he took up a standard of living far below what he was used to. This reflected a trend toward socialism among the Tachtigers; another Tachtiger, Herman Gorter, was a founding member of the world's first Communist political party, the Dutch Social-Democratic Party, in 1909.
Van Eeden also had an interest in Indian philosophy. He translated Tagore’s Gitanjali.