Colonel Gustaf von Heidenstam had tried in vain to change his son's aimless life style, and sent him to Paris to study art under Jean Leon Gerome at the Ecole des Beaux Arts. There Heidenstam boasted with his great plans, emphasized his aristocratic background, but actually did not show any ambitions to become an artists. After returning to Sweden in 1880 he married Emilia Uggla, a Swiss woman, against his father's wishes. His early poems Heidenstam sent to the Finnish writer Zachris Topelius, who encouraged him to continue. During the following years Heidenstam lived with his wife in Italy, France, and Norway.

In 1886 the Heidenstams leased a medieval castle in northern Switzerland. Among the regular visitors was the writer August Strindberg, with whom Heidenstam later disputed. In 1887 they returned to Sweden and settled on the family estate. His relationship with his father had improved, but facing an incurable illness Gustaf von Heidenstam committed suicide. He never witnessed the appearance of his son's first collection of poems, Pilgrimage and Wander Years, which became an immediate success in 1888. Nowadays its vision of Middle East as a place of exotic pleasures is considered bizarre. During the next decade Heidenstam's output was prolific. His first novel, ENDYMION (1889), was set in Damascus, and expressed the author's fear of the decline of Orient culture due to European influences. It was followed by a pamphlet, RENASSANS (1890), the prose-and-verse novel HANS ALIENIUS (1892), in which the protagonist, a wanderer, depicts his problematic relationship with his father, and DIKTER (1895), in which the declamatory and recitative tone do not hide Heidenstam's wishes to be respected as a national poet.

Heidenstam interest in Swedish history gave inspiration to his most popular novel KAROLINERNA (The Charles Men 1897-1898). It is a series of stories connected by the life of Charles XII, who is seen together with his patient and loyal soldiers at several stages of his career, particularly during the disastrous invasion of the Ukraine that ended at Poltava in 1709. Heidenstam's common soldiers occasionally express their belief in patriotic ideals, but the thoughts of officers are psychologically more complex. The author had visited the battle grounds in Poltava in the mid-1890s. Deeply moved he picked there flowers and sent them to the Swedish poet and diplomat Carl Snoilsky. Heidenstam saw in The Charles Men Karl XII as a tragic character and when the Kings sarcophagi was opened in 1917, Heidenstam was present.

Among Heidenstam's later works are a collection of poems, NYA DIKTER (1915), and an idealized childhood memoir, NAR KASTANJERNA BLOMMADE, in which he again wrote admiringly about his father. The book appeared posthumously in 1941. In the 1920s Heidenstam had designed and constructed a home at Ovralid, near Lake Vattern, where he lived quietly. "O Man, you will become wise only when you reach the summit of the evening-cool heights where all the earth is beheld." Heidenstam did not publish further works, and his physical and mental health gradually deteriorated. During this period his companion was Kate Bang, whose two children became close to him. Heidenstam died on May 20, 1940. Heidenstam's reputation has declined in the Swedish Social Democratic society to the point of eclipse. Strindberg, his adversary who also was an individualist, is considered a major writer. Later, after their break, Strindberg called Heidenstam "Sweden's most unintelligent man".