Born in Wakayama, Kumagusu was from childhood a memory monster — the kind of boy who memorized encyclopedias whole. He entered the preparatory school for Tokyo Imperial University but could not fit in and dropped out. At twenty-one he traveled alone to America, and for the next fourteen years wandered through the United States, Cuba, and Britain, becoming a world-class naturalist entirely through self-study. During his London years (1892–1900), he haunted the Oriental reading room of the British Museum, devouring manuscripts in eighteen languages — English, Latin, Chinese, Sanskrit, and more — and submitted over fifty papers to Nature magazine. But troubles, including biting a colleague, got him expelled from the museum, and after returning to Japan he retreated to the mountains near Tanabe City in Wakayama. His greatest lifelong passion was the study of myxomycetes (slime molds), for which he collected over ten thousand specimens and discovered numerous new species. In the 1910s he launched a campaign against the government's shrine merger policy — the forced consolidation of shrines that entailed clear-cutting the sacred groves around them. He argued that this was the destruction of both ecosystems and folk culture, becoming a pioneering voice for what we would now call ecological thinking. In 1929 he met with Emperor Hirohito (then Crown Prince) on a plant-collecting excursion and presented him with myxomycete specimens. His eccentricity and encyclopedic knowledge crystallized into a singular vision of the world he called the "Minakata Mandala."

A genius memory combined with inexhaustible curiosity, and a wild personality that cared nothing for authority, institutions, or convention. He was quick to anger and never backed down from a fight — but his love of nature and empathy for the vulnerable were absolutely genuine. The image of a man who commanded eighteen languages yet spent his days chasing slime molds through mountain forests in Tanabe is the perfect symbol of a unique being in whom vast knowledge and wild instinct coexisted.

His myxomycete research remains internationally respected, and the specimens he collected have become an important archive for modern science. His shrine-merger opposition campaign has been re-evaluated as a pioneering contribution to Japanese environmental thought. The Minakata Kumagusu Museum in Tanabe City, Wakayama, houses his achievements and vast collections, and researchers from around the world still visit.