In 1928 she was hired to collect plants in Mount McKinley National Park in Alaska, which yielded 6100 specimens. The next year she went to South America and travelled by canoe down the Amazon River, covering 4,800 kilometers in two and a half years, ending at its source in the Andes. This expedition resulted in 65,000 specimens. On that expedition she spent three months living with the Araguarunas, a native group in the Amazon. During this trip she was briefly accompanied by her contemporary Mary Agnes Chase. While in Ecuador, Mexía worked with the Bureau of Plant Industry and Exploration, under the Department of Agriculture. Her work focused on the cinchona or wax palm, and specific herbs that bind to the soil.