Alice Eastwood was born in 1859, in Toronto, Ontario. She attended public schools and a convent school in Canada. In 1873 her family moved to Denver, Colorado, and she attended public schools there, graduating from Denver East High School in 1879. Having been recruited as a substitute teacher while still attending school, Eastwood was hired on as a regular teacher at her alma mater. She would teach for the next ten years, all the while collecting plants and teaching herself botany using Gray's Manual and Coulter's Manual of Rocky Mountain Botany as guides. In 1890 Eastwood took a vacation to southern California, where she studied the plants of the region. The following year she worked for several months as an assistant in the herbarium at the California Academy of Sciences. She was offered a position as joint Curator with Katherine Brandegee in 1892, and left both Colorado and teaching behind for California and botany. In 1894, with the retirement of Mrs. Brandegee, Eastwood was advanced to Curator and Head of the Department of Botany, a position she kept until her retirement in 1949, at the age of ninety.
Eastwood was responsible for saving the Academy's type collection after the San Francisco earthquake of 1906. In contrast to the curatorial practices of the time, she had chosen to segregate the type specimens from the main part of the collection. This allowed her, upon entering the damaged building, to quickly and easily find the specimens and remove them to safety. The remainder of the collection was lost in the fire that followed the earthquake.
For some years after the earthquake, before the Academy had constructed a new building, Eastwood studied in herbaria in Europe and other parts of the U.S., including the Gray Herbarium, the New York Botanical Garden, the British Museum, and the Royal Botanic Gardens at Kew. Upon completion of the new Academy facilities at Golden Gate Park in 1912, Eastwood returned to the position of Curator of the Herbarium and set about rebuilding the collection that had been lost. She went on numerous collecting vacations in the Western United States, including Alaska, Arizona, Utah and Idaho. By keeping the first set of each collection for the Academy and exchanging the duplicates with other institutions Eastwood was able to build the collection, adding "thousands of sheets to the Academy's herbarium, personally accounting for its steady year in and year out growth in size and increase in representation of the western flora".
Eastwood published over 300 articles during her career. She served as editor of Zoe and as an assistant editor for Erythea before the 1906 earthquake, and started a journal, Leaflets of Western Botany (1932-1966) with John Thomas Howell. She was the active head of the California Botanical Club (of San Francisco) for a number of years starting in 1892 and participated as a member of many other botanical and horticultural societies. A member of the California Academy of Sciences since 1892, she was unanimously elected an Honorary Member of the Academy in 1942. Her main botanical interests were west American Liliaceae and the genera Lupinus, Arctostaphylos and Castilleja. She died in San Francisco on Oct. 30, 1953.