Eisen was born in Stockholm, Sweden, on August 2, 1847. He attended school at
Visby and later graduated from the
University of Uppsala in 1873. He came to California that same year to participate in a biotic survey sponsored by the
Swedish Academy of Sciences. He decided to make California his home and joined the California Academy of Sciences the following year. He was known to have diverse interests, including "art and art history, archeology and anthropology, agronomy and horticulture, history of science, geography and cartography, cytology, and protozoology, as well as marine invertebrate zoology" He is perhaps best known for his studies of oligochete worms and many species were named after him including those in the genus
Eisenia. In addition, he is considered to have been responsible for the introduction of the avocado and the smyrna fig to California and he wrote a detailed history of figs. He was a correspondent of
Charles Darwin and his work was referenced twice by Darwin in
The Formation of Vegetable Mould through the Action of Worms.
Mt. Eisen, in the Sierra Nevada in California, was named after him. ==World renowned==