Mexía began her career in botany in 1922 when she joined an expedition lead by Mr. E. L. Furlong, the Curator of Paleontology at University of California, Berkeley. Her successes started to mount in 1925 with a two month excursion to western Mexico under the auspices of Roxanna Ferris, a botanist at Stanford University. Mexía fell off a cliff, fracturing ribs and injuring a hand. Despite the trip being halted, it yielded 500 botanical specimens, including several new species. The first species to be named after Mexia, "Mimosa mexiae", was discovered on this voyage, and was dedicated to her by Joseph Nelson Rose. Various other species that she discovered were later named for her, including a flowering plant that is a member of the daisy family called "Zexmenia mexiae," now named "Lasianthaea macrocephala". She collected the type specimen of "Mexianthus" in December 1926, south of Puerto Vallarta.